[kps]

Not very similar. If you wanted a message, the OS put up a modal dialogue blocking everything else. The whole point of Growl was that it didn't interrupt you, and you could have its messages go away on their own.

[kergonath]

There were these yellow messages that popped up in the upper right-hand corner in MacOS 9. Those were just non-blocking notifications.

[thewebcount]

Exactly. They look a lot like Growl and the macOS X notifications and were not modal.