This is the first of a few posts I'll be making that have a focus on helping teachers with technology. Many of these will be relavent to everyone - but they will have an extra focus for the teachers.
My wife is an American History teacher and is working on building a set of powerpoint presentations for her classes. Pete's Powerpoint Station is a great site with a ton of pre-made, free-to-share powerpoints set up for almost all subjects including history, math, science etc. Under each subject, there are several topical powerpoints for different sections in your class. They won't take the place of creating your own powerpoints customized for your teaching, but they might help you get started and some of them might just be what you were looking for on a particular subject.