Team Fortress 2 Fan Site
Here's a short list of some of the more popular maps in the game
Dustbowl
One of the more popular maps in the game. Dustbowl is an attack and defend type map where one team has to capture two control points in order to progress to the next stage, while the other team has to defend their point until the time runs out. Many players have created their own version of this map.
Goldrush
A payload type map that was added in a free update shortly after the games release. One team needs to push the bomb cart towards the enemy base in order to win the match, while the other team has to prevent the cart from reaching its destination. It is often considered to be one of more popular game modes.