Neil Gaiman wrote Good Omens with Terry Pratchett. It's well worth reading. Neverwhere is interestingly weird but lacks the humour and what I can only decribe as Pratchettness. I haven't read any other Gaiman - he gets a bit gruesome for me.

While I've enjoyed Fritz Leiber, Zelazny, Peake and others, no-one is like Pratchett.

However, no-one's yet mentioned Harry Harrison's The Stainless Steel Rat