There are bookmarklets for all kinds of stuff. You can use them to do something as trivial as change the font size of the page you are seeing, to all kinds of CSS tweaks.
I’m gonna list some of the more useful ones here. I’m gonna try to credit wherever I got them from, if I recall it.
HTML Keyboard: Convert accented characters like Äñkít into their proper HTML sequences. Offsite, needs net connection. Provided by Fox Internet.
I don’t normally use bookmarklets for search, as Firefox search engines fulfil all my requirements there. My most commonly used bookmarklet is for posting stuff to del.icio.us. People who use it know what I’m talking about.