Theres really no good mod sites for the game, just the forums.
some good ones..
http://kerbalspaceprogram.com/forum/...p?topic=2680.0
http://kerbalspaceprogram.com/forum/...p?topic=4180.0
If you've bought the game, you can run plugin parts as well..
http://kerbalspaceprogram.com/forum/...24.0;topicseen
Helps you with orbital rendezvous.
http://kerbalspaceprogram.com/forum/...p?topic=8021.0
Does pretty much everything else.. takeoffs, orbits, landings.. Sweet stuff.