I've been teaching for the better part of four years now, in addition to living with a teacher for many years before that. I personally use Keynote 2-3 times a week and have gotten quite addicted to it. Beyond that, since I teach physics, I use Omnigraffle to throw together a lot of diagrams and figures. I also have a LaTeX Equation Editor handy to generate the equations for my slides. More recently I've started to use Pages to do my handouts and quizzes. Most of the papers I grade are submitted to me in PDF format so I can do all the markups in Acrobat Pro (I use a mixture of post-it notes and digital ink markups, using my Wacom tablet). It takes some getting used to, but doing the grading electronically can be just as fast or faster than on paper, plus you have a searchable archive of all your old papers and your comments, AND you don't have to carry around a stack of papers everywhere, just your laptop!
While not really "software", I am also very happy with my Logitech Presenter Remote (I think that's the name). Its a handy little black remote I use for Keynote presentations that has a built-in laser pointer and a countdown clock with vibrate alarms at 10- and 5-minutes-to-go. Whenever I go to a conference, the timer is a real blessing. Its just something I'm always sure to have in my laptop bag.
