I've just got myself a very cool add-in for Firefox called Hyperwords. This is what I've been looking for for ages. Select a bit of text and you get a pop-up menu which will take you anywhere on the internet you want to go - Wikipedia, search engines, shops, Technorati, you name it. Select a number and you can do currency conversions on it, you can tag things in delicious.

The menus are configurable, so you only get the functionality you want, and there's all sorts of clever tailoring you can do as well. 

Well worth a look for Firefox users.