expresso and vows are the most popular options. Although there are many good options out there. Update: As of Nov./2011, Mocha is the successor to expresso, according to the Readme.md at the expresso repository. should.js is often used with Mocha to allow BDD-style assertions.
Whatever you choose, I recommend using gently to stub core or third party modules.
I use/maintain testosterone, a little testing framework that runs test on serial so you can abuse gently. Not so many people using it though.