DOOM3: where should I go for mods/maps???

I just recently bought Doom3 for PC and was wondering where the best places are to find other levels and/or mods. So far, I haven't tried any mods or home-brew maps. What sites have them? Also, what mods are cool and what suck. I'm also not quite sure how to go about loading a new map (in the game, or before it loads up???).

Any info would help out this Doom3 newb.

I haven't really played D3 since it came out, so I couldn't tell you where to find downloadables... That being said make sure you check out the Quake 3 map someone remade. It's the inital starting map, the really small one.

thanx