GitHub use it for their download server - try running wget against one of the URLs they provide for downloading a tarball, you'll see it redirect to nodeload.github.com, hang for a few seconds (while they generate the file) and then redirect you to the download location.
http://github.com/blog/678-meet-nodeload-the-new-download-se...
http://transloadit.com/ are the most interesting Node.js startup I've seen.