One of the few books that had me laughing out loud was Lem’s Star Diaries. I was reading that on the train, and my seat neighbour greatly added to the amusement, when he tried to get a look at the cover. Apparently he was too shy to just ask.
Another one was Neal Stephenson’s Reamde, which had one particular chapter that just cracked me up. Overall, I found it to be the worst of his writings so far, though. I gave it away after I was done with it. If you want something fast-paced and light-hearted instead, start with Snow Crash and Diamond Age. The Cryptonomicon is pretty sombre, but is a good prelude to the the Baroque Cycle, which features a number of hilarious chapters.
And for something entirely different: Wilt, by Tom Sharpe.
Also, if slightly humorous or grotesque will suffice, some of Umberto Eco’s works can be considered too: Baudolino or Numero Zero. Also not to forget: Michael Morcock’s Dancers at the End of Time. And come to think about it, Adolf Muschg is also funny to read. He’s got a talent to play with words that rivals or even surpasses that of Terry Pratchett. As with Pratchett, much will be lost in translation, but if you read German, then check out at least Der Rote Ritter and Sax.