Ubiquity, a new Firefox plugin from Mozilla Labs offers a really nifty way to get a variety of Web services from any page. Unfortunately, its text based interface will probably limit its appeal to people comfortable working with a command line, but it really is worth a try.
Once the plugin is installed, you type ctrl-space (otion-space on a Mac, which alse requires you to install the growl Firefox extension first) to bring up the Ubiquity window. Then the fun begins.
For example, you can type the commpand "w firefox" and get a listing of Wikipedia pages on Friefox. Select some text or an image on a Web page and type "email this to steve" and if steve is in your gmail contact list, the content will be dispatched. Mozilla is working on adding other mail clients, including its own Thunderbird. Highlight an address and type map and up comes a Google map with that location.
For Ubiquity to become, well, ubiquitous, is will need a better user interface. But if you are interested in experimenting, it's well worth a try.