OpenStreetMap Quick Check:

http://vatavia.net/mark/osmqc/

"A quick and easy way to help improve the OpenStreetMap database"

If you can look at a satellite photo and answer a simple question, you can help.

The first example project is categorizing residential road endings. This system could also be used for many other purposes.

Feel free to edit/discuss the wiki page for this project:

osm.wiki/QuickCheck

I have been using my OSM viewer for a while now both on the desktop and on my Windows Mobile phone. The two versions have interface differences, but share much of the code.
The mobile version uses my phone's GPS for several things:
centering the map
recording where I have been in GPX files
uploading my current location to my web server so my wife can see where I am

It can also download my wife's location so I can see when to head downstairs to be picked up.

I am planning to package this software for non-programmers to try on their own desktop and/or mobile device, I just need to think of a good name for it...

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.

Ideas for a new plugin that will allow MapWindow (open source .Net GIS) to communicate with OSM are being posted at:

http://www.mapwindow.org/phorum/read.php?2,9197,9217