What's all this, then?
Built into Mac OS X, but often neglected, is the concept of 'services'. They are small and targeted tools you can use in almost any program to insert text or do something useful with the current selection. Select text and Google it or look it up in a dictionary. Select a mathematic expression, like "6 * 7", and calculate it. Just select text (or place the insertion point where you want it) and pick a service from the Services menu in the application menu.
Normally, you have to write services using Apple's Carbon or Cocoa frameworks, using languages like C, C++ or Objective-C. ThisService lets anyone with a basic handle on programming choose their own language (like AppleScript or Ruby) to write a service in, lowering the bar for entry, and making certain tasks easier by letting you use the right tool for that task.
Okay, but I don't program.
Even so, a bunch of other people have been able to write useful services thanks to ThisService. We provide links to their fantastic work.
Help! My Services menu is all crowded.
ThisService is solely for creating new services, but the free Service Scrubber is a marvelous tool to clean up your Services menu with. (No relation.)