I have several adventure seed ideas over at
http://www.obrienscafe.com/adventures.html
These aren't fully developed GURPS adventures, since that would require me being a serious publisher with a serious licensing deal. But they were all written with the intention of being played with GURPS.
If you're willing to invest a bit in a setting, Transhuman Space has really excellent mini adventures under the heading Personnel Files. Totally worth checking out, and designed so that you can run them in about four hours.