The DiscWorld series of books by TerryPratchett are incredibly funny, well worth reading, fantastic, fun page turners; you wish they would never end!
As a bit of a background, the setting is a large disc that sits on the backs of four great elephants (The Fifth Elephant had a nasty accident), these in turn stand on the back of a turtle.
As well as some magic and fun fantasy, it explores some rather unique physics that makes this series attractive to your average ScienceFiction reader.
It's also attractive to anyone that likes a bit of light-hearted social commentary with their humor.
There is also a very popular Discworld MultiUserDungeon. For more information, go to http://discworld.atuin.net or start playing immediately at telnet://discworld.atuin.net/