I use My Tracks when I'm on the bike, so I can find out where I was. I haven't used it a lot yet, but from what I understand, you can use the files it generates in Google Earth to replay your route. I like that it shows elevation changes as well as where you've been. It even records your location when you don't have cell service, which often happens when I ride. I find it handy when I'm with a group (and not leading) so I can see where we went during the day.
For real GPS duty, I use Copilot, which has its own maps, so I don't have to rely on having a cell signal. (OOH! Look! There's a map update!)