No more cut and paste - I often have to replicate several pages of a content management system (CMS) that require me to enter tags in a field one at a time. I used to enter them on the first page, then copy the text, open Text Edit or Stickies, paste the text, then move on to the next page and flip back and forth between screens while re-entering tags on each page. Now I just snap a screenshot of the tags I've entered and use it as a reference on the remaining CMS pages.
File those registration receipts - After I've bought an app, I snap a shot of the receipt screen and store it in Evernote. Of course I also keep the copy the vendor emails me, buts it's nice having a backup in case my email client flakes out.
Make audio editing easier - While editing podcasts in Audacity, I often need to note the exact time of a clip. While it's easy enough to make a note with pencil and paper (remember those?), a screen grab lets me see at a glance exactly what other settings I was using at the time.What are some of your more unconventional uses for the screen grab tool?