NumSum isn’t new, but I hadn’t really played with it until today (it’s actually nice that things are slowing down for Thanksgiving, giving me time to check out stuff I’ve missed).
It’s well thought through Ajax spreadsheet application, with very basic functions. They’ve added tagging, and you can generate a permanent URL for any spreadsheet, even without creating an account (non-registered spreadsheets will only be up for seven days, however).
There’s also a great tool for importing the spreadsheet into a blog. See this post by Adam Marsh for an example.
It’s more of an experiment than a truly useful application. I do not believe you can make spreadsheets private, for example. However, seeing stuff like this and writely make it extremely clear that a full office suite on the web is not only achievable, but should be here already.