The latest public version available I know of was available at Cassini for Framework 3.5, but the link is dead (all links pointing to Dmitry Robsman's blog are dead in fact, and he was Cassini author I believe).

However if you know the file was named Cassini-v35.zip, you can now find it here: https://larytet-master.googlecode.com/files/Cassini-v35.zip for example.