SuperCalSuperCal™ is a visual display calibrator capable of measuring and correcting most conventional displays, including LCDs, CRTs and projectors. SuperCal doesn't require any hardware measurement devices - only your eyeballs - yet it can be much more accurate, based on how well you pay attention to what your doing

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This is an invaluable tool, especially for PowerBook and iBook owners. After spending about 15 minutes going through the configuration, you can get a color profile that puts the default mac included profiles to shame. Colors look much more vibrant and accurate with the profile I made with this on my PowerBook.
TigerLaunchTigerLaunch is an easy-to-use and easy-to-configure application launcher for OS X. It displays an Apps menu at the top of your screen listing all your applications in alphabetical order—no matter what folder they appear in. You can easily configure it to exclude applications you rarely or never launch.
I'm suprised no one has mention this one. It's a really light application launcher that isn't cluttered with extra crap.
iTermiTerm is a full featured terminal emulation program written for OS X using Cocoa. We are aiming at providing users with best command line experience under OS X. The letter i represents a native Apple look and feel of the program interface, and an emphasis on complete international support. iTerm was merged from two projects, CTerminal and TerminalX, both of which were based on JTerminal project. The current version is still in beta stage. It is however very much functional and usable.
Best part of this terminal replacement is tabbed sessions.