AppFreshSoftware updater is great for Apple applications, but appfresh has the ability to check thrid party apps for updates. A great little tool for keeping upto date.
Although applications are easy to uninstall on a mac there are usually small preference files hanging about. Appzapper cleans up an uninstall all of those files.
Disk Inventory XA simple program that scans your disk and visually shows what files takes up how much space, good to visualize if you are running out of space and you have some size hogs.
Grand PerspectiveVery similar to Disk Inventory X, shows you what files take up how much space on your website.
QuicksilverOne of the best applications ever made for the mac. Its a quick launch tool that enables you to open and do practically anything. Very fun and really increases your productivity.
StuffitAn application design to unstuff or unpack nearly any file. Create if you use archives a lot.
TextWranglerA very cool text editor that lets you edit text files quickly. Sometimes more powerful than text edit.
TidyUpAn application that enables you to search through a list of files finds duplicates. Great if you have many files and you need to organise them.
TodosLeaves no application behind. A very visual way to view all of the applications on your computer. Kind of surpassed by the stacks in the dock.
UnRarXUnrarX gives you the abilty to easily uncompress .rar files. Very good, although stuffit can now handle .rar files.
XslimmerGive your mac a diet. It goes through and removes the powerpc or intel parts of your program. It can really reduce the size of applications. Can also break them if you are not careful.