GPXSonar is a prgram which allows you to take your GPX files on the road in your Windows Mobile/Pocket PC based device. You use it to view GPX files downloaded from grocaching.com or exported from GSAK. Notes can be recorded while geocaching and on return home you can export them to GSAK for your reference in the future.
Ref: GPX Sonar Home Page
April 2008 - This link seems to no longer work....
Compatability Issues with Windows Mobile 5
Some users hare having problems running GPXsonar on newer windows mobile 5 devices. Apparently the Config folder is missing and your field notes will not save. The fix for this is to find the folder you installed GPXsonar in and create a Config folder. (Usually /program files/gpxsonar/config) Then you're up and running again - ready to hit those caches.