There are lots of lists on various blogs of people’s favorite OS X software (some listed below).
One of my favorite little programs for OS X is SnapNDrag, which is a superior screen capture utility. While OS X comes out of the box with easy screen capture to pdf, that doesn’t do you any good when you’re trying to embed an image into a web page. SnapNDrag supports capture to PNG, JPEG, TIFF, and GIF, which makes like easy. Check it out.
Some lists of essential OS X software:
http://www.43folders.com/2007/04/27/my-menubar/ (thanks, Bob!)