You'll find things like primordial sketches (which are truly horrid -- enter the 'shock' factor), or the concept and reference art I use to drive the strip forward. There's marker art in there, and even the occasional single-panel comic (although those are pretty random, and not necessarily Schlock-related at all).
For those of you NOT subscribing to KeenSpot PREMIUM, don't worry. Schlock Mercenary will remain free, and you don't have to rummage through the gallery to understand the strip. In fact, I still spend far more time supporting the FREE stuff here than the PREMIUM content (by at least an order of magnitude). Just this Saturday I spent six hours cranking through nearly a week's worth of strips. I love you all so much that I drew until my hand started to hurt.
Of course, now you've got one more reason to subscribe to KeenSpot PREMIUM: Not only is it free of ads (including the popups that we now have to run to pay our bandwidth bills), and not only do you get the screamingly fast and blindingly convenient WarpKeen service (your favorite Keen comics all on one page), you now get to peer into the Schlock Mercenary scrapyard and see blueprints, prototypes, and lots of unerased pencil marks.