If you're into geocaching (GPS-guided high-tech treasure hunts), I'd recommend GPXSonar (
http://gpxsonar.homeip.net/default.aspx) for decoding and displaying GPX files downloaded from Geocaching.com.
If you just want to know where you are and some Points of Interest nearby, Pocket Streets (free with Microsoft Streets and Trips) does a good job.
When you decide to buy GPS navigation software, stay away from Arkon's MobiNavigator. It's a rebranded iGuidance package, so I figured I'd support Arkon since their hardware is very good. From multiple emails back and forth with their customer service, it looks like the version I bought (their first and only version) will NOT be made WM5-compatible despite the fact that iGuidance has a patch that makes it so.