If you're the type of person who uses a lot of search engines and doesn't mind having a toolbar installed, you might want to check out Firefox Super Search. This Firefox-only add-on puts the power of 160 search engines in one toolbar, with about 70 that can be searched right from the toolbar itself.
If you're used to Firefox's built-in search box, this is a pretty big step up both from selecting one engine at a time and having to add additional engine tools from Mozilla's catalog. It's also laid out in a really straightforward manner, putting search engines with which you can search (from within the toolbar) in drop-down menus on the right side of the query box. There's also a directory of the other 90 or so engines that require a visit before beginning a new search.
My only qualm is that you cannot reorganize the order of the engines or make a shortlist of your favorites. It's also missing out on modern search box niceties like auto-complete and search-as-you-type suggestions--two things that require the search box to know which engine you're searching with first. On the plus side, though, the curation and organization of the engines is spot-on.
Note: This add-on is experimental, which means that it may not work perfectly in your browser. We used Super Search on Firefox version 3.5.1 without issues.