Applejack is your friend if you can't boot up a GUI on your mac. Once installed, if you have a problem, boot into single user mode by hitting cmd-S and typing 'applejack'. You're then presented with a prompt which lets you repair disks, permissions, cashes and remove your swap file. Thankfully, I have never needed to test this in a real situation, but it's a comforting thing to have installed on your system.
Proximity is a neat little app which automatically runs Applescript when a bluetooth device approaches or leaves your bluetooth-enabled mac. For example, if you set it to pair with your phone, on approaching your mac you could have it run an applescript to sync your calendar and address book for you. Alternatively you could use it as a proximity security tag, so on leaving your machine the screensaver triggers and becomes password-protected, and on approaching it returns to the desktop. For useful scripts, check out this blogger's code.
WolfQuest is a free game with an educational slant developed for Minnesota Zoo. You take on the role of a wolf and have to hunt and find a mate. It supports single or multiplayer play, and in the game attempts to be as realistic as possible, down to the body language of the wolves when communicating. A special scent-view mode lets you find food and fellow wolves.
My major frustration with safari was that it could only support Google as it's search engine, and there was no way to change this without editing Safari's coding, a risky process that crashed safari if your modified version didn't have exactly the same file size as the original.
Main Menu is a small app that sits in your menu bar and provides you with easy access to a large number of maintenance tasks for your mac, such as force emptying trash, cleaning cashes and logs, and repairing permissions and indexes. It also lets you create scheduled tasks and execute these every set period of time for you. Everything it does is posted to you via Growl.
Bricksmith lets you create lego models using your mac. There is a large array of parts available and once you get the knack of assembling these in 3D space you can make very realistic looking models. The highlight is probably the Minifigure generator which builds lego people for you - perfect for creating a forum profile image of you which stands out.
Processing is an open source Java-based development environment that aims to make it fast and easy to program prototype software (each project is known as a "sketch"). While the basic syntax is of a Java nature, many libraries have already been created to make programming using Processing simple and fast.
