You can't really download it in a standalone form, legally. Jabo doesn't allow his plugins to be distributed outside of the Project64 package, so you'll just have to install the latest version of Project64 from http://www.pj64.net to get his three latest freeware plugins.
Depending on who the primary author is, you could even count Project64's RSP plugin as a fourth Jabo's freeware plugin, but that was made by both Jabo and zilmar.