Growl 2.0.1 (Mac OS X) | 6.3MB
Growl is the is ultimate notification system for the Mac. It's the easiest way to know what is going on with other applications while you are working on something else.
One day we were working with pictures in iPhoto while waiting on an important email to arrive. Every time an email came in, it interrupted our iPhoto work when we switched to Mail. We had no idea what each email was, we just had to keep clicking back over to Mail every time there was a new email. At the end of the day we decided that we needed a solution to this problem, and thus Growl was born.