The Java source code for rssNimbus (in the form of an Eclipse project snapshot) is now available from this link. Ultimately, it should be moved to an on-line version control site like Google Code, but for now a big blob is what you get. See the ReadMe.txt file inside the archive for more info.
You can find details on installing and running rssNimbus in this blog post. As a note to developers, you should use the Google Eclipse functionality to install your version of rssNimbus on Google App Engine instead of the command line tool as referenced in the blog post.
This source code is being released under the Creative Commons Attributed-Share Alike 3.0 license, which means you can pretty much do what you want with it as long as you give me credit and you make changes available under similar license terms.