PopClip is a Mac OS X extension that “appears when you select text with your mouse on your Mac.” You can then select from several options including copy & paste, and actions like search, spelling, dictionary from a little popup that appears over the selected text. It is kind of like what happens when you select text in iOS.
It is extendable and PopClip extensions have been created for many common tasks and services. For example select some text that has a URL and you will be presented with the option of opening that link you your browser.
There are many extensions already written to incorporate PopClip with many common apps and tools. There is also extensive documentation for those that wish to create their own extensions. For example I am thinking a nice summer project would be to create an extension that searches a specific Bento database. A use case might be to be able to select a student’s name from an email, then pull up the parent contact information from within a Bento database. Will see if I can get to playing with this this summer.
You can read an exensive write up of the tool over at AppStorm. I have been using it for a while and find it quite helpful.