Coral p2p web cache, which seems to have reached the useful stage.
Thanks Green Hat Journal, from both of us :-).
And on another academic note, I'm quite surprised that Xen is the virtualization technology gaining traction with the enterprise linux companies. It seems like UML and/or coLinux would be more likely given UML's track record with ISPs, and/or coLinux's cross-platform + low overhead. I thought the Xen contributors must have sold their legal souls between getting the MS Research funding and signing a Windows source code license.
Anyway, I'm hoping that we reach what I see as the next stage in virtualization that open source could really benefit from -- the downloadable data center. Where instead of distro or OSS support companies providing ISO's and package management, they'll just serve up great big VM blobs that have been preconfigured for solving a particular data center need. I believe it's the logical next step for appliances, and Oracle has already starting doing for it's RAC trials.
Open source wins big for ISV's bundling up a set of virtual appliances, because the licensing picture for all parts of the stack is soooo much simpler (no lawyers to call!!!) than with commercial companies, as I mentioned here back in 2001.