Certain Twitter plugin or script

Hey everyone, I am wondering if anyone here can help me with a twitter plugin or script of something I saw a few months back. I completely forgot what its called and I’m not even sure if it’s a plugin.

Basically, what it does is if you have like a free ebook or something, you can have visitors download it–but in order to download it, they have to tweet a specific message about it.

I can’t remember where i saw it so if anyone can help me out, I’d really appreciate it!