Growl Fork is a free version of the notification system Growl!. Note that it's based on the old version 1.2 and so lacks some of the newer features in Growl!.
Growl Fork runs works in OS X 10.7 Lion, but adds nothing that isn't in the premium version of Growl! Growl Fork is compatible with most major programs such as Skype, Spotify and iTunes and opens a small dialog window in the top right of your screen when there's a new notification.
There are quite a few customization options and you have a large amount of control over how you want Growl Fork to look such as the size, position and duration of notifications. The developer site is however very limited in terms of support but informs users of the latest updates to Growl Fork which are frequent.
If you've switched to OS X 10.7 Lion and don't want to upgrade to the paid version of Growl, Growl Fork is a very solid replacement.