Neil Gaiman is
phenomenal. Don't read him expecting Pratchett, though--he's very different, though they have similarities. I started with
Neverwhere and then
American Gods, which are both fantastic, and his most recent
Anansi Boys is awesome. You might also try his collection of short stories,
Smoke & Mirrors, which is full of really amazing stuff.
I'm also a big fan of his Sandman series of graphic
novels, which starts with
Preludes & Nocturnes--not the best volume in the series, because Gaiman and the
artists were all finding their stride while doing a very, very unique month comic, but the last episode is still my favorite out of what I've been able to read of the series. Neil Gaiman also collaborated with Dave McKean on the recent fantasy film
MirrorMask (he scripted and produced, McKean designed and directed), which has had a very,
very limited release, but which is really good.