1. Omnigraffle, for diagramming and doing up plans etc.
2. CoRD for Remote Desktop Connection to Windows boxes
3. Apple Remote Desktop to control other Mac boxes
4. Perian plugins for Quicktime.
5. BURN, for cooking optical media, if you still do that sort of stuff/have an optical drive.
6. XCode 4.2, for good times/compiling/all the GNU toolchain (trust me, you'll need it).
7. ClusterSSH, for manipulating more than a few boxes at once via terminal.
8. Macports, for when you just want it to magic-compile, or you need a UNIX port of something nobody else uses/hasn't seen for ages.
9. Growl, for event notifications/bouncers.
10. MagicPrefs to allow all the *really* cool freakshow capabilities of your trackpad and magic mouse. It does some amazing stuff.