Must Have apps? Here is my list:
MenuMeters - puts all sorts of useful information up into your menu bar, such as CPU load, disk activity, memory usage, etc. The CPU stuff is invaluable
iClock - adds color, fonts and full Day, Data and Month to the menu bar clock. Looking at mine, I can see that it is "Mon 26 Jun". It also adds goodies like world clocks, a menu of currently running programs, which acts as a task switcher, a calendar app, etc. etc. etc. It is a fabulous bit of shareware.
Path Finder - a serious file manager for serious file management. I use Mac's Finder for simple day to day stuff, but when I need to do backups, detailed reconfiguration of folders and so on, I pull out the heavy armor, Path Finder. You won't believe how many additional functions it adds, which makes file management activities SO much simpler.
It is also a replacement desktop for the Mac, providing WAY more functionality than the standard desktop. You don't have to use the desktop feature, and it is off by default, so don't worry that it will take over your Mac if you install it.
Classic Menu - this little application lets you build up a menu of available applications in a cascading men fashion, rather like the Windows Start menu, or the launch menus in almost any Linux window manager. Plus, it can change that boring blue Apple symbol on the upper left of your menu bar to the original vibrant multi colored apple - this is worth the shareware fee all by itself! I know that I can achieve cascading menus via the Dock, but I just LIKE them on the menu bar better!
Thats it for my "must haves".