The Chub - By Soren
The Chub is by far one of the most popular and most mod-able frames out there at the moment. If you were to strip it down to it's bare essentials it's got lots of space to build off of, looks great, and is pretty damn pose-able too.
Ironsnpier has a wonderful variant here.
Joshua Newman has a killer reskin here as well.
The Conscript/Commisar - By Soren (again)
Both the Conscript and Commisar are very similar builds and you probably wouldn't even notice the difference between the two at first (I know I didn't). It's another sleek looking frame from Soren with a lot of modding potential. Those arm or leg pieces can easily be swapped out with studded bricks and the backpack potential is near unlimited. You can get the LDraw file over at Dinvull.
MgN-302/303 Series - By Malcom Craig
An older, but very versatile frame originally designed for Mechaton. It may not be as pretty as some of Soren's stuff, but oh man this guy is a modder's dream.
Jed September has countless variations of this guy as well as a few breakdown shots, this one is by far my favorite.
You can also get the LDraw file for this one over at Dinvull.
More still to come!