I can’t believe I haven’t written about this already. Along with Quicksilver and Adium, this may be one of the more useful applications you will find for OS X. Personally, I don’t find it amazingly necessary, but others will argue otherwise.
This is essentially the notification program which should have been built in with OS X. It covers most programs, from Adium to Mail and Safari, and will notify you of a vast variety of different events occurring within each program. For example, if someone connects on Adium or if you get new Mail. Very nice indeed, and when combined with a variety of styles and plug-ins for greater compatibility, its definitely worth a download even if it’s not vital.