Windows only: Open source notification system Growl for Windows has finally hit the 2.0 milestone, and with it comes a ton of new features that let you further customise the way you receive your pop-in corner notices.
Among the new features are images in notifications, playing sounds when notifications are received, keyboard shortcuts, history and notification summaries. There is also support for pushing notifications to iPhone, email and Twitter, as well as pulling notifications from other machines on your network (whether they are PCs or Macs). There are more applications that can integrate with Growl for Windows, with more being added all the time, so the real benefits of this upgrade depend on the enthusiasm of developers.
Growl for Windows is a free download, Windows only.