While it's relatively immature in terms of widgets available, I like yui3 (NOT 2) for building application type sites. It has a strong component model and the a

While it's relatively immature in terms of widgets available, I like yui3 (NOT 2) for building application type sites. It has a strong component model and the attributes/events system and widget base are excellent and I believe it's the only one that's really serious about progressive enhancement.