Practically nobody knowed it, but JOSM had a possibility built in to be launched as an applet. This was implemented many month ago. Now I finally got myseld into fixing the last problems (including a bit of server hacking) and made a demonstration setup.
You can either use the standard login or create your own at http://www.openstreetmap.de. Note, that this is currently a test server and so your entered data, including your account, is thrown away after some time (unless we decide otherwise). On the good news, feel free to browse around, it will not disturb the main server in any way (hopefully).
I also setup a Testserver API of the upcoming Version 0.4 (the Ruby on Rails thingie Steve is coding on). It runs in development mode, so a) data may be resetted frequently and b) it is not a clever target for doing benchmarks.
Testserver API (0.4): http://www.openstreetmap.de:3000/api
Try it out! It won’t bite you.