Mac: There's nothing wrong with OS X's Terminal, but iTerm 2 adds a bunch of features you probably didn't realise you needed. With awesome things like autocomplete and a traversable timeline, it makes your time spent at the command line much more productive.We've mentioned iTerm here and there in the past, but never really gave it its due. iTerm 2, currently in beta (but pretty stable), is a big improvement over the original. There are plenty of features that'll speed up your command line tasks and fix frequent problems. It does things like highlight the cursor when it's lost, provide instant replays of what just happened, copy without resorting to the mouse, gives you a split pane view, and a ton more.
If you're just vacationing in the Terminal, you probably don't need the power iTerm 2 provides. If you use the Terminal frequently, however, it's enormously useful.