clearcase

Version control system, from Rational software (http://www.rational.com/)

Also one of the things I'm responsible for at work at the moment; I range between not minding it, and wanting to shred the disks it lives on.

Basically, imagine CVS on steroids .. then beef it up a little more.

I am also responsible for this piece of... software. It is quite easy to branch code with this tool and ludicrously easy to merge large branches back in. However, to use the full functionality of ClearCase you also have to purchase ClearQuest - something that Rational don't make clear in their marketing...