Check out Bruno's stuff, since he's much more into it than I am.
Resources:
Mac OS X:
- GPS Connect, which can download waypoint data from Garmin devices via an USB-to-serial converter.
Windows:
Generic UNIX/Linux:
- GPSDrive and Roadster, Gnome applications
- gpsd, a TCP daemon to echo GPS data in a more palatable format.
Series 60:
Multiplatform:
- GPS Babel - cross-platform data importer/exporter/converter.
Real Interesting Products:
- Tom Tom Go - the only really good GPS device I've handled so far, and which has been recently enhanced.