A couple of years ago, I created a fill-in-the-blanks percentage calculator for my own personal use, but I put it up on my site in case others might find it useful as well.
With no advertising and very little publicity, it now shows up close to the top of Google's search results for queries like "percent off calculator."
I slapped some (kind of obnoxious) advertising on it ... but for HN users, here's an ad-free version:
http://pressbin.com/tools/percentage_calculator/?HN=
Just make sure to enter something for the HN variable -- your HN username, some random characters, a URL-encoded haiku, etc.