Last Friday I was talking about a document rating system, which used Ajax. It came about following a meeting with the client where we discussed ways to get the most useful content to the user when they opened the homepage. One solution was to show the most often accessed documents and the other to also show content ranked by the user-driven ratings.
Never having implemented a document rating system I suggested it to the client partly because it was a solution to their needs and partly because I fancied the "challenge" and always like to do something new in each project, rather than churn old the same old techniques over and over.
You can see a working demo and download the code on the link to the Komodo Media site. Later this week I'll have a working Domino version online for you to play with and download.