I only discovered OSM a matter of weeks ago and was delighted to find that about the same time I had an idea (which I discarded at the time as cloud cuckoo land thinking) for a free world-wide map built by thousands of GPS users, somebody with more dedication, imagination and free time was actually starting to do it!
I soon found a big hole in OSM right around where I live (Idridgehay, about 9 miles NW of Derby) and have steadily been filling it in using GPX data collected on foot, bike and driving. I'm glad to see someone else (Paul Todd I think) has started doing the same. As the weather improves I will be adding more public footpaths locally and in the Peak District.
I have a long-standing interest in maps as well as GPS and programming and have developed my own software (britain.poco.org.uk). Having discovered OSM I have added the capability to save GPX data in PocketBritain running on my mobile phone which can then be imported into JOSM. I am currently adding the capability to take photos using the phone's camera within the program which get GPS locations. The next step will be to allow these to be fed into OSM or Freemap.
I think OSM could be as great as Wikipedia and would like to see it grow way beyond the simple street map content of Google maps and suchlike to provide more information than we get in OS maps but free and worldwide. I've seen growth in just the few weeks I have been using it and think what we have now is just the start.