I pretty much use prototype.js in every web project.
That is why I’ve forked prototype and started trimming it down. At the moment it’s almost half its original size (Sizzle took up a lot of code) and it still passes all its tests, albeit with a few minor changes: e.g. you need to quote values in CSS attribute selectors:
$$( 'foo[bar="baz"]' )
$$( 'foo[bar=baz]' )
You’ll find my fork on GitHub