Linux: Ubuntu 9.04 introduced a set of upper-corner notifications similar to Growl, but they lack for customisation in both appearance and usability. Mumbles aims to bring a true Growl-like experience to Linux desktops.
"Aims to" in this case, because the apps that can use Mumbles each require a plug-in to do so. At the moment, it's a pretty nice collection of apps—Firefox, Thunderbird, Amarok, and Twitter's API, among others—but definitely needs some rounding out. The app itself allows for changing the stay-on time of notifications, offers a few themes for desktop integration, and stacks the notifications and responds to clicks, unlike Ubuntu's built-in notifications. If you happen to be working on a network with Growl notifications sent out, Mumbles can pick those up, too.
I had trouble getting stacked apps into a screenshot, hence the use of the official image, but it did respond to Firefox downloads after installing an add-on.
Mumbles is a free download for Linux systems only, and mostly for Ubuntu 9.04, which is the only platform officially tested.