GIS apps (Score 1)

While this may not specifically address mobile GIS/GPS, one very good GIS Linux application that I use occasionally is TNTlite, which is part of a group of programs from MicroImages. (I have _no_ association with the company.) It's not opensource, but is is free for personal use, and is somewhat similar to GRASS mentioned above. (Actually it runs on Unix/Linux, Mac, and win.)

http://www.microimages.com/tntlite/

OpenMap is another free application of interest, although I've found it to be a little quirky.

http://openmap.bbn.com/whatis.html