The SourceForge Most Active throws up some good stuff. CrystalSpace caught my eye - having been playing with 3D environments quite a lot recently. It looks good (check the screenshots); plenty of features, and seemingly very well designed.
The out-links are less reassuring. VOS is well-intentioned but doesn't seem to have thought much about the flaky network infrastructure and will have difficulties scaling (whereas Proksim/Quazal have already been there and back). A too-common warning sign was this: "The syntax of VOP is based on XML. It is not 100% strictly valid XML... It borrows the nested element/attribute model and syntax, but deviates from strict XML when necesary or convenient". Uh-oh.
Where do I go to say this one again? XML is good not because it's clean and efficient - plenty of times it's gnarly and crufty and painful. XML is good because everyone agrees how it works - with all the cruft. Because of public agreement, rather than because of elegance or cleanness, there's a wide playing-field of supporting components (free parsers, off the shelf, behaving pretty much as expected). And because there's a commodity infrastructure, we get innovation on top (this stuff, for example).