Having used Prototype + Scriptaculous, jQuery and Ext, I'd say jQuery is a good generic library choice. Libraries these days are fairly interchangeable in terms

Having used Prototype + Scriptaculous, jQuery and Ext, I'd say jQuery is a good generic library choice. Libraries these days are fairly interchangeable in terms of core features, but I find idiomatic jQuery tends to be shorter, more intuitive, and overall more popular (which is good if you plan on leveraging existing plugins for those sorts of things that browsers really should do but don't). My experience with Ext hasn't been so great because I feel the learning curve is a bit steep, there are a lot of leaky abstractions and whenever I had bugs, they were both difficult to trace and to fix. Also, I feel deviating from the defaults tends to be cumbersome (or impossible) Prototype has improved immensely over the years, but imho, the improvements are also its downfall: historically, the Prototype API has been pretty unstable, with methods moving from one class to another and some APIs being completely overhauled. One other library that I should mention (that I haven't used much myself) is Dojo as it is the "Java of Javascript" in terms of domain specific libraries. There's stuff ranging from crypto to comet there. Pick your poison, I guess.