Frantically scrabbling for answers..
Just had a panic since I use WP a lot; but it appears this is not a plugin.
It's a tool that some theme developers use for convenience, it's just a PHP script that lives in the same folder as the images you want to thumbnail. You don't install it separately since it is part of the theme itself.
Oh.. and I see a lot of people saying that if you have problems with it not generating thumbnails you should open the permissions in that folder to 777. Humm.
Have a look here; a forum posting where someone asks if doing that is a good idea: