There’s a number of standalones, as well as some that have the usual cast of characters but can be started without prior knowledge.
@kingannoy’s suggestion of Going Postal is one of the latter. (And if you saw the Sky production, well, the book’s better.)
As to the standalones:
If you like gentle mocking of organized religions, try @GulliverFoyle’s recommendation, Small Gods.
If you’re into Egyptology or mathematics, try Pyramids.
If you like soldiers or scrappy young feminists, there’s Monstrous Regiment.
If you love the Blues Brothers movie as much as me? Soul Music
Just never start with The Color of Magic, unless it’s 1983. (And don’t get me started on the movie!)
Yeah, I know, this is all off-topic but I can’t help it when it’s one of my favorite tangents. 
