For those not experienced with D20/3.5, prestige classes are classes that have requirements and must be taken later in the character's career. The majority of such classes exist in Open Gaming License supplements put out by third party publishers. As a result, there never will be a shortage of classes that most people have never heard of yet are within the rules allowed to be taken by any character that fills the requirements no matter how strong they are.
It's very common practice for such supplements to provide base and prestige classes that are just normal classes fitted specifically for the setting, or producing character classes that are in most aspects not too fun to play or lack any creativity. These five classes presented are interesting enough for me to take interest in, and I would generally suggest any of them to a player wondering what prestige class to take.
5. Sacred Bander (pg 108-109 in Thieves World Player Book)
The Sacred Band are masters of organized tactical combat, and fight in pairs with their oathsworn partner in battle. While most examples of Sacred banders in the novels paint them as homosexual, it's made clear in the Sacred Band class description that this is actually not the case as both males and females may take up such an oath and the pair of banders need not actually be romantically engaged, though examples exist. The most interesting part of this class is that in order to take the class you are actually required to have somebody else in the party also have the Sacred Bander class and be your partner in battle.
The most fun that this class has to offer is the handful of mechanical benefits both Sacred Banders receive. For being adjacent to each other and being of higher levels the banders get bonuses such as being immune to flanking, AC and damage bonuses, even a bonus to medicine on their partner. My big issue with this class besides the fact you need two players, but that is easily the most entertaining aspect of the class as well.
But for a large high-level campaign the class is a perfect addition to the group and ensures that your forces can stand right in the front lines. The issue is besides the small teamwork bonuses, there is not much reason to pursue the high levels of the Sacred band. The last useful ability is gained at level 8, there is nothing too neat laying at level nine and ten.
4. Black Powder Avenger (Pg 20-22 in Ravenloft: Heroes of Light)
Of all of the prestige classes in Heroes of Light, The Avenger catches my eye because of simply how much raw utility power the class can provide. The master of firearms, BPA’s may dedicate a single weapon with his vengeance to give it up to a +3 in attack and damage which likely stacks with any enhancement bonuses the weapon itself may of had already, but does not stack with special ammunition bonuses. In addition to five free firearm feats over his career the BPA also has the ability to fabricate his own ammunition and explosives for rather cheap, later being able to supply his friends with masterwork ammunition.
He eventually gains an apprentice that can carry his explosives and provide support fire as a cohort. Not many classes are given such a follower usually, but it is nice to know you if your powder monkey gets eaten you can train another one and eventually turn your cohort into a fellow full fledged Black Powder Avenger like yourself. The biggest drawback to this class, the requirement of reloading, is negated at level seven as the Avenger learns to reload as a move action. But the absolute best part of being an avenger comes at level nine with the ability to do a massive burst of damage at the expense of temporarily breaking your dedicated weapon. A critical with such a shot is easily enough to send most foes to their graves.
3. Assimilated (Pg 27-29 in Dragonmech Core)
Some mechanics get so deep into their respect to the great machine spirit that they decide to fuse themselves into the mech itself. Effectively, you are a Dreadnaught. The requirements for being Assimilated however are over-restrictive. The main issue is the set of skills you require, 10-15 ranks in four rare setting-specific skills are required, which is absurd for a prestige class. Two are knowledge which are not hard to get but in particular mecha piloting skill is needed at levels too high to be possible by only a rare handful of classes. In 3.5, the only feat I could find that would make this easier was Able Learner in Races of Destiny but if you don’t get it at first level then you are boned.
But that hitch is just a matter of asking your GM if you can be overpowered, or taking levels in Mech Jockey. Then the obvious problem arises like where are you going to get a level 10 techpriest to help you connect you to the mech, and where you are going to get the mech you are going to be fused to. Which I admit is a pretty good way to restrict the class. But the requirement that makes me sad is that Assimilated are true or chaotic neutral and act like an impartial computer, the class itself only states non-lawful but in order to be assimilated you have to suffer from lost humanity and thus be rendered a neutral alignment anyway. It makes me so mad I want to exterminate, EXTERMINATE.
Once fused to your mech, your body shrivels and loses physical ability until you finally are fully in control of the mech you now will pilot for eternity. On the bright side, you also become an absolute monster in combat. It’s hard to argue with +15/+10/+5 as a large living construct. The letdown is once you take the plunge you cannot take levels in classes you cannot physically perform the duties of. This becomes mostly DM opinion, especially because the class claims you cannot be a fighter because you are physically incapable of fighting which I call foul on, your mech is fully capable of being fitted with a sword and shield and going to war in the name of the EMPRAH!
2. Ironbound (pg 32-33 in Hammer & Helm: Guidebook to Dwarves)
Like other powerful classes, it’s way too easy to dismiss the Ironbound by its name and picture. The Ironbound is encased in the best of exotic dwarven armor and excels on the front lines as a defensive fighter. Ironbound get a few great bonuses most classes only dream of, for starters over the course of leveling as an Ironbound you get no less than five free proficiencies in Exotic armor types and an inherent AC bonus that stacks with your armor bonuses.
But what about the large amount of time it would take to sleep? Heavy armor is infamous for the amount of time it takes to get in and out of! Well, at level 3 you get so tired of climbing in and out of it that you learn to just get used to sleeping inside your armored home. At level 7, you get completely used to it. And not only can an Ironbound ready a shield as a free action once per turn but you can even dual wield them at level six. Why just use one tower shield when you can have one in your other hand too! You may waddle at .5 miles per hour but being damn near invincible is worth it.
1. Harpooner (Page 64-65 in Broadsides! Naval Adventuring by Living Imagination)
You are probably thinking the same most people think when finding out about this class. A Harpoon? How much damage could you possibly do with that? Surprisingly the Harpooner is not only the best class in the book but is the most open to interpretation class I have ever seen for one good reason: There is no standard stat-block for Harpoons. However the only hint given is that the Harpooner class requires both Weapon Focus and Exotic Training in Harpoons. The 3.5 Arms and Equipment guide lists it as a large D10 weapon that if pulled from a foe does the same amount of damage as when it went in and assumes the weapon is tied to rope held by the user. The Harpooner class has this same ability but gets a extra bit of backstab esque damage and can pull the harpoon free automatically as a standard action (It's never stated whether or not he can do this at range, but I assume he can). At level 7 the extra withdraw damage becomes a whopping +5D6 on top of effectively doubled initial damage
But it gets better. As any seasoned old school DnD Player will tell you, if the GM is sending you somewhere with water then you are going to be facing terrible gigantic underwater creatures he never gets to use otherwise. The Harpooner was clearly designed with this in mind and gets a +1 attack bonus for each level he has towards aquatic creatures of huge or larger size and at 10th level can spend a day to charge one harpoon with the ability to force any one aquatic beast to save versus death. This is made even more entertaining as the Broadsides book also lists an expensive magical Harpoon that gets the exact same effect and the class does not factor in if his imbuing ability stacks with such an harpoons effects. In short, this class is designed to take down Huge/Gigantic Sea Creatures with extreme prejudice but also operates as a dangerous ranged damage dealer regardless of what he is fighting.
As ten rank prestige classes go, The Harpooner is amazingly powerful and capable of doing immense damage. The classes requirements really are not too steep either, and as exotic weapons go the Harpoon really is not that hard to find in any setting where an ocean exists. Really any character with the right skill set for throwing a Harpoon should consider this class, especially if you know the GM owns lots of bestiaries that are bound to have aquatic monsters in it.