The two things I found the most useful were Twitter and Blogger.
I had used it Twitter before but in a very limited capacity. Learning how to search for stuff and using hashtags improves the Twitter experience immeasurably, as does Tweetdeck (desktop client) and Ubertwitter (Blackberry client)
I'll definitely continue to use this and I hope that the other 23 things people continue to use the #ox23.
Blogger is also good, very easy to use and a good way of recording things, for example progress on a special project at work. I love the gadgets that you can add to your blog in order to integrate everything.
Honourable mention goes to Google Docs, I was really impressed with what you can do with it, especially the forms.
Less useful to me were things like Wikipedia, YouTube, Facebook etc as I'd already used them quite a lot.