I have been working on an OSM tile downloader/cacher/viewer application in VB.net. It works pretty well now and the source is available through subversion:

http://svn.mapwindow.org/svnroot/OSM_VBnet/

I post about this program in a thread on the MapWindow forum at:

http://www.mapwindow.org/phorum/read.php?2,9197,9477#msg-9477

In addition to caching and displaying tiles, it overlays GPX files (tracks and waypoints) on the map.