I'm a big fan of my browser's contextual menu. This menu, which can most easily be pulled up by right-clicking your mouse is commonplace for really useful extensions that are polite enough not to clutter your browsing experience with extra keyboard shortcuts or UI.
Firefox extension SimpleSearch is a really good example of this concept. Once installed it lets you search from a handful of sites like Wikipedia, IMDb, Google Maps, and dictionary search engine Definr. You just highlight the word or phrase you want to search and do a quick right-click to hop to the results.
The one downside is that it requires going an extra level deeper in the contextual menu compared to your browser's built-in search option, which in Firefox and Chrome defaults to Google. But for that additional step you're rewarded by not having to copy and paste into your browser's search box or open up a new page. In future releases I'd love to have the search options moved to the main right-click menu instead.