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+1 for YUI

+2 Grok Douglas Crockford's DOM theory then optimize how your scripts use the DOM, it is very easy to abuse the DOM, don't. Grok the DOM.

Sprockets looks promising if you're using Ruby. Haven't had a chance to use it in a project yet. http://github.com/sstephenson/sprockets

Already mentioned, but django-compress if you're using django:

JSMin is also good.

Gzip?

Apache should do this.

Better yet, use Google Page Speed.