Jeff Atwood posted up a C# implementation of Identicons the other day. While I do not see them being all that useful, they are pretty neat. I whipped up a little [csl] Add-on that can display an Identicon.
The code is more or less right from Jeff; I modified image caching to use [cs] caching rather than the straight up HttpRuntime Cache.
I currently use this as a replacement link for the comments permalink in my blog (it used to use '#'). To see an example of these in action, just make a comment on this blog post. Once your comment appears you should have your unique Identicon!
This [cs] Add-On is packaged up into the CarKnee Bundle.