The in-browser search box is now ubiquitous. It can be found in every major browser, and some--like Firefox and IE, allow you to both change which engine you want to search with (from a drop down list), and add more engines from a large directory.
It's fast and easy, but a new Firefox extension called Lookpicking goes one step further, essentially giving you little packs of search engines that you can call up like bookmarks and share with others. More importantly, it lets you swap back and forth between your entire collection by typing in just the first letter or two of the engine you want. If you've ever used Launchy, QuickSilver or any other desktop application launcher, the idea is similar.
This has many potential uses, with the single most one being a shared home computer. If you're on a machine with several people sharing one user account you can have multiple sets of search engine packs loaded up, and whoever is using the browser can simply use their selections.
It's also useful in academic environments. If you're doing Web research in several different disciplines you can swap back and forth between a huge list of search engines. Lookpicking lets you pick from any of the ones you've added once you start typing in the query. Then, once you're on that page you can continue to search from its index before switching off to another site's search.
One thing to note is that this has been classified as an "experimental" Firefox add-on, so if you want to skip the Mozilla user registration to download it, you can grab it here (firefox 2 users click here).