I’ve recently StumbledUpon just about the best list of free Mac software I’ve ever seen: 100 Incredibly Useful & Free Mac Apps by Mac.AppStorm. Very well organized, this list includes many apps that I’ve not heard of but have since added to my computer. If you scroll through the comments, you’ll find some more recommendations, including several strong recommendations for Alfred, as a QuickSilver replacement (as much as I loved QuickSilver, I’ve since returned to using LaunchBar, as QuickSilver isn’t quite reliable on Snow Leopard).
100 Incredibly Useful & Free Mac Apps | Mac.AppStorm
