The first major expansion to Catan is a literal one. The Seafarers of Catan pushes the borders of the play area to encompass the ocean surrounding the main island and introduces ships. Like roads in the main game, ships are used to connect settlements on separate islands through shipping lanes. Seafarers also introduces a new land hex: the gold field. Instead of being limited to receiving only the single resource type represented by common lands, these valuable hexes bestow any resource of choice to those fortunate enough to control them. A new antagonist also appears in the form of pirates who can steal resources and prevent the construction of new shipping lanes.
While standard games of Catan can be played by randomly placing land tiles, the nature of Seafarers with its broad ocean simply can't work that way. Land and ocean tiles would be a scattered affair and could result in several insignificant and inconvenient islands that might be too difficult to reach. To prevent this, Seafarers comes with a book of eleven scenarios which dictate board setup as well as provide special rules or new victory conditions. While the quantity of material within provides many options for play and even some truly epic possibilities (one scenario requires a second copy of the original game), there's a bit of charm lost when restricted by these elements. Seafarers, while interesting, doesn't really bring much that's new over the basic game and is one of my least favorite expansions.
If Seafarers can be said to have brought breadth to the world of Catan, this second major expansion gives it a new depth. The Cities and Knights of Catan doesn't mess around with new lands but focuses entirely on making the mainland more vibrant and complex. Cities, no longer merely built to receive bonus resources and some points, are now the focal point of the game. This expansion introduces city improvements and commodities. Like resources, commodities come from hexes during the die rolling phase of each turn but are awarded only to those players with adjoining cities. These commodities are spent to improve the living conditions of your city in merchant, academic and military aspects. The more you improve a city, the more likely you will receive progress cards (this expansion's replacement for development cards from the main game). Improve it enough and it will become a metropolis providing many benefits.
So, that's cities, but what about knights? Instead of gaining soldiers through development cards as you would in the basic game, knights are built with resources and have new abilities but must be kept fed. They can be used to block an opponent's road building progress, force the pesky robber to move to another hex or to keep another knight too occupied to use their own abilities. Knights are also crucial in defending Catan from a new villain: Barbarians.
With every roll of the dice, there's a chance that barbarians sail closer to the island's shores. If, when they arrive, there are more knights able to fight than there are cities, the assault is repelled and the player who had the most active knights is rewarded with another precious point towards victory. If the forces are insufficient, however, those who participated least in the battle will have a city pillaged and reduced to a settlement.
This is easily my preferred method of playing Catan. The number of methods by which you can work towards victory helps it achieve a balance that can sometimes be lost in the basic game if players get cut off from necessary resources early on. Also, the constant dread of a barbarian attack keeps players on their toes and offers great opportunities for cooperation and backstabbing. It's downright criminal that Microsoft has yet to create this as DLC for the original Catan XBLA game.
One of the major issues with Catan's expansions is that they can be a little pricey. Both Cities & Knights and Seafarers cost as much as the original game does. At that kind of money, it can be hard to justify the purchase. But fear not, there's still a couple of sets available that will bring something to the table without much expense.
The first is The Great River, a set of three additional hexes originally given away in an issue of Games Quarterly Magazine. Using this variant, cities, settlements and roads positioned along the special hexes earn Gold Points which translate to a Victory Point at a rate of three to one. It's not much of an add-on, but it can be interesting to engage in a mad race to reach the river delta tiles. And, at about $5 on the interwebs, it's a valuable addition to any Catan game.
Another inexpensive way to shake things up is the Event Card set. One of the common problems with playing a game of Catan is that, while the resource numbers are set up according to probability, die rolls don't often reflect that. I've seen many games where sixes or eights hardly come up at all despite being the most likely rolls in the game (minus seven, of course). Event Cards eliminate this problem, by providing a deck of cards which represent every possible combination of die rolls. To maintain at least some level of randomness, five cards are removed from play with every shuffle, keeping it so every die roll doesn't come up every time.
In addition to the probability fix, many of the cards feature special events that reward players who are doing well or help out others who haven't been as successful. You don't have to play using the events (the cards are useful enough just replacing the dice) but they're an interesting way to spice up your game. Also at around $5, this is another great buy for a more casual Catan player.
If you play Catan for a while and decide that you're really hardcore, however, you might consider looking at some of the scenario expansions. They haven't been published in English yet, and it seems unlikely that they will be, but translations do exist on BoardGameGeek and other Web sites.
The largest collection is Das Buch. This 192 page book with accompanying pieces features fifteen new scenarios and eighteen variant rule sets, including rules for volcanoes, castles and player combat. There is also a lengthy history of Catan included for those interested (and happen to read German). Considered by many enthusiasts to be the definitive set of expansion material for Catan, it is out-of-print and copies often run in excess of $50.
Another set of German-only expansion material is Atlantis, a collection of older expansions and variants republished and packaged together. It includes the aforementioned Event Cards as well as few new scenarios. Some of the Das Buch materials are revised and reprinted here, though it's a smaller package overall for about the same price. If you're only going to get one, get Das Buch.
If you're totally batshit nuts for the game, as I am, why not go all out and get the 3D Collectors edition of Settlers of Catan, featuring hand-painted ceramic tiles and pieces. The Cities and Knights expansion is bundled in with the main game, but it'll cost you a cool $300. God damn if it isn't gorgeous, though.
As you can see, there is much, much more to Catan than meets the casual XBLA player's eye. And some of these could easily be integrated as downloadable content. If they were willing to do it for Carcassone, I can't see any reason why it hasn't been done yet for Catan.
If you haven't played Catan on a tabletop using some of the expansions, you really have no idea how much there can be to this game. If you haven't played Catan at all, shame on you. You're missing out on one of the greatest games made in the 20th century.
MOAR ON THE TABLE