Here's a few Javascript links which give quick access to a few website searches, and other misc. stuff (emailing links to the current page, f'rinstance)
You should be able to just drag these links up to Safari's bookmark bar and use them. You can treat them just like any other bookmark - put them in a folder, whatever.
These were originally posted on my techstuff blog and mentioned on MacMentor, but putting them here, where javascript: URL's work makes it easier to add them. JavaScript and browsers can be a little finicky about punctuation, so it's easy to screw them up if you enter the code by hand
If you look through the code for the bookmarklets (either by viewing the bookmarks in Safari, or the source of this web page), you'll probably get an idea of how it works. With a bit of work (and let's not forget, trial and error) it should be possible to modfy this to search a whole bunch of other sites.
Chris