I really enjoy Douglas Adams, Terry Pratchett, Grant Naylor(two people going by one name, Red Dwarf writers), Tom Holt and Robert Rankin.
Rankin is particularly good if you've got a bit of a warped mind and don't mind some very childish sex jokes.
If you're a bit of techhead/geek/nerd then try Neal Stephenson. His best one is probably Snow Crash (come on, the main character is called Hiro Protagonist...).
Spider Robinton is interesting but a little difficult to get in to for some. There is a lot of backstory you need to read before most of the characters make sense.