Mine is the cover of the limited 6th ed. Call of Cthulhu rulebook, which (physically and at this tiny size) looks like a blasphemous tome from H.P. Lovecraft's stories. HPL is one of my favorite authors, CoC is one of my favorite games, and I love books -- so it seemed like a good fit. At first, I wasn't sure how I felt about it being book-shaped rather than square, but over time I grew to like that aspect of it.
I have changed it once so far, and after a couple of days I missed the old one and changed it back. I also feel like I should keep a consistent avatar until I've been posting here for a bit longer.
As has been mentioned, they're a great tool when you're just skimming a thread -- and it's always interesting to speculate about why someone chose their particular avatar.