I just recently discovered The DM of the Rings, which explores what Tolkien's classic work might look like as a D&D campaign with a bunch of easily-distracted players and an exasperated DM.
Lord of the Rings is more or less the foundation of modern D&D. The latter rose from the former, although the two are now so estranged that to reunite them would be an act of savage madness. Imagine a gaggle of modern hack-n-slash roleplayers who had somehow never been exposed to the original Tolkien mythos, and then imagine taking those players and trying to introduce them to Tolkien via a D&D campaign.
The comic is assembled entirely from movie stills, and hits all the right comedic notes. Great stuff.
Via The spectacularly obtuse blog.
Two more nice RPG comics can be found at Giant in the Playground, both silly and fun: there's the graphically impressive Erfworld, a parody in the most overwrought high fantasy tradition, and the minimalist stick-figure comic The Order of the Stick, an AD&D "metagame" comic where the characters argue over xp, stats, and rolls.
EDIT: Now that I've gotten deeper into reading it, I just had to amend this post to add that the Order of the Stick is the best thing I've seen in a long time. LOVE it. I wasn't nearly enthusiastic enough about it at first, but it's really something.
Reading these, I was reminded strongly of that other geeky favorite of mine, 8-Bit Theatre, the epic webcomic that retells, more or less, the story of the original Final Fantasy game in a highly irreverent fashion, taking swipes at roleplaying, rpgs, and the whole fantasy adventure genre along the way. (I originally mentioned the comic in my "Excellent webcomics: strips and dailies (Part 2)" post.)
This is part of the first strip. Looking back, even though it's all based on the same remixed 8-bit graphic set in all its primitive glory, the graphic design has come a long way in the current strips. Creator Brian Clevinger has certainly had a lot of practice, with over 900 strips and counting.
Dear me -- revisiting the archives, I've just realized that this is in fact my third time posting this comic. And the last, I promise. With a bookmark stash like mine, there's no excuse for reruns.