There's plenty of ways to get mapping on the go these days but still not that many that offer OS mapping. Viewranger is a standalone mobile app that's not supported by a desktop program like Anquet and Memory Map but it's very powerful, and the new generation of smartphones make that less of an issue.
Anquet maps have been around for a good long while and version 6 of their software is more robust and fully featured than other, earlier incarnations that we've tried. Interaction with a Windows-based smartphone or PDA is simple and the prices are getting more appealing too. The interface can still be a bit slow and clunky at times though.