Partly because of my own project, Tracks, I'm interested in seeing how other people implement todo type lists. So I signed up for a Ta-da list to see how they were doing it. I was pretty impressed. It's intended to be a fairly simple system to tempt people into signing up for a full Basecamp account, so there are no categories, due dates or anything like that. However, they do have some really neat ideas.
One thing it and Flickr showcase is the power of XML-HTTPS Request to narrow the usability gap between ordinary applications and web-based applications. I'm going to look into the documentation for it to see if I could use XML-HTTPS in Tracks.