I frequently take screen shots of error messages, dialog boxes, informational messages, web page receipts and more. I use OS X’s built-in <Apple-Shift-4> to initiate it. I then choose the area I want to take a picture of. At this point, Sharpshooter intervenes and lets me rename the picture, send it to the trash or move it to a directory other than my user-defined screen shot output directory. I’ve been doing this since… well, my screen shots go back to 28 August 2007.
The latest wrinkle is that I decided it would be convenient to have these screen shots available on any of the machines I use (or even via a web site, in a pinch) so I recently changed my default screen shot output directory to a directory under the control of Dropbox. Now my screenshots are available virtually anywhere for me.