Well, there's this if you ant to do that, but if you don't and want to do this other thing then there's this other thing for that.
And of course, this, that an the other thing are god for this, this, and this other thing.
Seriously, what do you want to do with your Mac?
O a helpful note, i find these to be pretty universally useful:
Menu Everywhere - lets you use a mouse button to pull up the active apps menu, puts a menu bar on top fo a second screen, and other useful things
Synk - keeps your data in sync in two or more folders - great for backups
Cinch - puts windows into full screen or half screen by dragging
TotalFinder - lots of useful finder enhancements including tabs on the finder window