Harmony is a procedural drawing tool which provides some interesting drawing possibilities; Google Sphere and Google Gravity reconfigure the world’s most famous search engine (make sure you actually try a search in both!). Or So They Say is just a lovely ambient experience, but possibly the best known (although it’s not clear exactly what his role in it was) is The Wilderness Downtown which is a music video for the band Arcade Fire. Built in HTML5, it combines specially filmed footage with content from Google Earth to make something a bit mental. I suspect we’ll be seeing a lot more of this sort of thing as HTML5 support gets better.
You may find a lot of these projects aren’t compatible with all browsers, especially the Google experiments. I recommend SRWare Iron, which is a version of Google’s Chrome browser with all the stuff that talks to Google secretly stripped out. Isn’t open source great?