There are a lot of applications available (most of them free) that do all sorts of neat things. So far, I've been playing around with ElJay (an LJ client), the ixMAT scanner (a barcode-scanner), Maps, Sky Map and the ubiquitous Doom. I installed my own ringtone from an MP3 file (which I cut to size with Ringdroid). I set up my email account. Using the browser is a breeze. I even used it to make a call!
However, the firmware I had on my phone when it came fresh out of the box was 1.1 -- and we're up to 1.5 now. Normally you'd receive the updates from your provider OTA (Over The Air), but I was getting tired of waiting. There was one app that I really wanted (Astrid) that I wanted to have, but that I could not find in the Market. I suspected that doing the upgrade would solve my problems -- perhaps Astrid utilises some more advanced OS features...
So after hunting down the two image files and by finding the right incantations, I manually upgraded my phone to the latest version. And lo and behold: there was Astrid!
The new version has a lot of other cool features as well, like taking a video and uploading it directly to YouTube. How cool is that?