Probably didn't make it clear enough, it's repeatable so you'd use Twitter as the service and whatever password you usually use (master password), it'd come back with a hash and you use that as your Twitter password. Whenever you need your Twitter password, you repeat the process.
https://github.com/simontabor/pw - it's got node-webkit so there's a mac app in that repo and should also work on Windows + Linux nicely (this is simply to take it out of the browser where it's easy to lose tabs and put it in a clean, small window)
It's a nice touch to provide a local application to run as there is no way people should be doing this in the browser. You might want to remove the tracking from the local app though, even if it is for analytics purposes, I don't want something that deals with passwords like this to be phoning home.
I may do, it's not too difficult for people who care about it to just take it out - I'd actually rather people take the code, remove tracking if they'd like, edit colours/whatever and make it their own. It's more the concept that I care about (never ever send your main password over the wire)
https://github.com/simontabor/pw - it's got node-webkit so there's a mac app in that repo and should also work on Windows + Linux nicely (this is simply to take it out of the browser where it's easy to lose tabs and put it in a clean, small window)