On the server, ASP.NET and the free version of SQL Server. On the client, the Yahoo UI library, which I highly recommend, and a lot of hand written javascript. But I tried to avoid being too fancy with AJAX, so I used it only for things like editing in place and voting without reloads.

The only fancy AJAXy thing I have is the bookmarklet. Which is kinda cool, if I may say so myself :) I haven't seen any other web site use something like it.