http://www.tt-foundry.com/ under GPL
They're simply awesome! very detailed for the scale the author(s) have used. Do you know if they have been made using pixel-art or 3D modeling techiniques?
Btw, TT Foundry is supposed to be a graphics repository as g.s.c., isn't it?
As far as I know handdrawn; anything else is nearly impossible with the 8bit palette TTD uses. And this is only from one person, not a general repositiory.
In that case, the merit is twofold...the quality of those pictures is awesome.