Mountain Lion’s Notification Center [sic] ⇥

This is better than Growl, insomuch as it offers a layer “underneath” the desktop for missed notifications. The execution leaves much to be desired, however, due to its strange mix of Helvetica (used in the Centre) and Lucida Grande (used in banners). I have a hunch that the final version will ship with Helvetica Neue as the default system font. It looks better on high-resolution displays, anyway.