Well, I just recently made the switch to Mac and I must say I love it. I wanted to get a post going for different aps that people have found for mac's that are useful and fun.
You Control Tunes ITunes controls in your applications bar
BackLight Use your screensaver as a background
Visage Customize boot images and text
SideTrack Customize your Mac laptop touchpad
Share your fun tweak and customization programs.