Core item: Diving Rapier, always
The most recent update to Valve's free-to-play MOBA Dota 2 brought in-game player-created guides to help people out. When a guide is selected, the recommended items in the shop will change to those that the player has selected. Also, upon each level up, the recommended next skill will be highlighted for your convenience.
Dota 2 is a notoriously hard game -- probably the hardest MOBA to learn -- so this will be a great boon to players. This will definitely decrease the amount of times I have to alt-tab in the middle of a game just to find out what items and skills I should be getting, at least.
Some heroes don't have guides pop up in-game though, due to zero recommendations from other players. So get out there and rate guides you like!