On of the best sites for maps and mods is ThinkingWithPortals.com. Consider that at the moment there are few mods because the SDK has been just released and modders need time to create something different, but there are available already many maps.
They announced a couple of weeks ago winners of Summer Mapping Initiative, a contest for map makers that had a lot of entries:
Over the course of the past two and some change weeks, our 13 judges
have spent over a collective 140 hours playing 240 of some of the best
custom maps this community has seen. As I have said previously, the
turn out for this contest has been amazing and I want to thank each
and every one of our participants.
In addition you can find also something at Mod DB.