Keith Laumer's
Retief! (free!) has already been recommended, but I'd like to add that Baen also has more humorous books from him,
The Universe Twister and
The Lighter Side (free!). There is also
Retief's Peace by William H. Keith Jr. which is very much in the Laumer's style and extremely funny.
Another Baen book that geeks will probably enjoy is
Wizard's Bane.