every time i write myself a bookmarklet to modify a webpage in some only-maya-cares kind of way, i experience the user-extensible computing environment people claim to want. i can't hear your complaints about javascript i'm too busy living in the future
i wonder if bookmarklets would make a good intro programming project -- I've gotten a *ton* of mileage out of just "grab elements from the DOM, iterate through them, check a condition, and delete/hide/style them if they match" in different contexts.