One Page Rules is an initiative to create simple and (mostly) free, miniature-agnostic rule sets for sci-fi and fantasy games. Grimdark Future is their rules for WH40K-like games.
The basic rules fit on one side of an 8 1/2 x 11 sheet with special rules on the back. There are also army lists for the various army types and full rulebook PDFs sold for a modest fee ($5). They also have lines of 2D and 3D miniature designs you can purchase and print out.
So far, they have large-army sci-fi and fantasy rules, sci-fi and fantasy skirmish rules, a space combat game, and arena combat rules. Cool stuff!
Here's a run-through of the Grimdark Future rules.