BrashFink has made a fantastic original comic book by taking screenshots from Half Life 2 and laying them out inside funnyubook panels, with speech balloons, etc — it's like static machinima, or a fotonovela made with game graphics.
(Thanks, Olly!)
Update: Brian sez, "Gamics.Com also has a *ton* of comic book-type stuff that is done from screenshots of various games, along with nice tutorials on how to DIY."
Update 2: Patrick sez, "I noticed yesterday a new piece of software for Mac OS X that lets you make comic-style layouts with bubbles using your own photos."