I've recently subscribed to (the free version of) ScreenCasts Online based on some comments I read somewhere. This is a video podcast that explains tips and tricks for the Mac, and presents third party software — either commercial or free — in great detail, which is ideal if you are planning to purchase some specific commercial program.
The typical show starts by presenting a problem to be resolved or by directly talking about the specific program to be presented. It is then followed by a detailed inspection of the user interface and some sections that exemplify common tasks. At the very end, it gives pointers to either fetch or buy the program. I have to confess that I find some of these shows to be excessively detailed, to the point of becoming boring at some point. But they are still a good way to see all the possibilities a given program can offer.
"Thanks" to them, I've fallen in love with OmniGraffle and OmniPlan ;-) Pity they are so expensive because I won't be paying that amount of money for my extremely modest needs.