What can I do with Quicksilver (OS X launcher)?

Quicksilver is far more than a lean application launcher, though it can be used solely for that purpose. You can set up triggers (keyboard shortcuts) to append to text files, open files, mail selected documents, increase the text size of selected text, navigate clipboard history, etc.

Note that there will be not insignificant learning curve, but it is well worth it.

The Quicksilver documentation page makes a good jumping off point. The tutorial page is a good place to start on how to configure and then customize Quicksilver, in particular the screencasts bring the functionality to life.