DropBox

A great thing to have if you use more than one computer. This will create a folder on each of your machines, and keep them all synchronized. Save a file to the Dropbox folder at work, and it'll be downloaded automatically when you get home. Awesome for saving things like address book data and calendar information, or 1Password keychains. It works seamlessly behind the scenes and is fully secured by SSL.

Screen Recycler

If you have an older computer or laptop, this app will turn them into extra secondary monitors. You can even convert Linux and Windows machines. It works over the network, using VNC, so you can even go wireless. Great for coding or email, but not so much for video due to the slower refresh rate.

ControllerMate

Use drag-n-drop flowchart logic to control any mouse, keyboard, joystick, or pretty much any input device for the Mac. I have a USB audio device with a big volume knob, and ControllerMate let me change it into a giant mouse scroll wheel. Endless possibilities.

SubEthaEdit

This is my text editor of choice. It has all the features a web developer could want, and yet is still very light and fast. It has syntax highlighting for all the major formats. Multiple views of the same file can be created to edit different parts of a large file simultaneously. It It lets you switch or convert back and forth on the fly between character encodings and line ending types. These are the small things that you really miss once you don't have them anymore.

But the thing that really makes SubEthaEdit special is the ability for multiple people to edit the same file at once. You can even do this over the internet, complete with iChat integration. Edits made by each person are highlighted in a different color. The highlighting comes in handy even if you are editing by yourself, by making it easy to see what has changed at a glance.

Versions

A beautifully designed Subversion client. It takes the often used features of SVN and makes them a prominent part of the user interface, rather than buried in the menu system like some other SVN clients.