openjsan is a open library modeled over CPAN (or PEAR, if you’re into PHP). It’s a colection of loosely related javascript-libraries, to which anybody can contribute. The result is a bit messy, but interesting nonetheless.

MochiKit looks very solid library. The author comes from a python background, so if prototype is js for ruby, MochiKit must be js for python. It looks very coherent and is grounded on unittests en masse. It’s infrastructure is compatible with both dojo and openjsan, which means that you can draw in code from thoose libraries.

buRRRn core2 looks very promising aswell. Like dojo, it’s aimed at multiple platforms (It was initially coined for use in ActionScript), and it comes with a unittest framework (ASTUce). There’s an impressive test-coverage for the framework.