I’ll my tupense worth as well into the fray.
No-one mentioned any Iain M banks yet (or just plain Iain for such classics as the wasp factory, a bit sick in parts I’ve been told but I just found it enthralling) so I’ll put in excession as it’s a lot of fun to read.
Other Sci-fi (pronounced sciffy by Iain Banks himself I’ve been told [book reading by Ian Rankin who drinks with Mr Banks every so often, so I wasn’t exactly told personally]) faves of mine include Peter F Hamilton (the reality dysfunction trilogy will keep you going for a while approximately 3000+ pages of fun), although I’d suggest starting with something smaller of his like Mindstar rising to see if you like his work, I also liked Battlefield Earth by old loony tunes L Ron Hubbard (before he got religious).
Fantasy wise the already mentioned Tad Williams is a good author and I particularly like his memory sorrow and thorn trilogy as well as the otherland stuff.
Crime-wise / comedy for the British out there I strongly recommend Christopher Brookmyre, although for the non brits it can be difficult as it can be dialect orientated (not as bad as Irvine Welsh). He writes in a similar style to Carl Hiaasen (but not set in California) if anyone reads his work (who’s also a Insomniac recommendation!).
For forensic science stuff I recommend Jeffery Deaver (the coffin dancer is simply amazing in my opinion)
I’ll also add in there Ian Rankin, who’s Rebus character has improved my whiskey knowledge no end.
Just comedy based I’ll add Practetts mate Robert Rankin, the Armageddon trilogy is especially fun to read .
Hopefully I’ve not overly repeated what others have said (I did skim everything else that had been posted)
I’ll also tack at the end that these are personnel choices and therefore may not stand up to the scrutiny of others, but hell I don’t care I read em, re-read em ands further re-read them again coz I love em. I cannot over empathise how much I love Christopher Brookmyres work, if you read this and take one bit of advise from me read his stuff, you’ll love it. In fact you can read a short story of his on the net for free at
http://www.brookmyre.co.uk/bampota.htm
Give that a go, you’ll like it. Trust me I’m a man with a lot of free time on his hands from lack of sleeping I know a good book when I find one!