Commercial software and its free, open-source equivalents
Go here for a great list of traditional (read: expensive) commercial software and the free, open-source programs you can use instead. The Webi page includes links to the home pages of all the FOSS (free, open-source software) programs it cites.
Two I plan to try are Cinepaint and Paint.NET, both image editors.
Many of my favorite apps are missing -- but the fact that there are enough FOSS apps that you can miss a bunch and still have a credible list is a very good thing.
Still, what's nice about this list is that it includes apps for Mac, Windows and Linux. I've always said that the best way to experience open-source is to do it on the OS you already know. Then the transition to a free, open-source OS like Linux will not be so daunting.
Free, open-source software is important for many reasons, but one of the biggest for me is that it enables me to compute with a clear conscience. Let's be real, most of us are using PCs with pirated software. Even if Microsoft Office, Photoshop and what have you are made by big corporations who charge many hundreds of dollars for their products, that's still no justification for stealing them. I feel a lot better using software that's meant to be free -- and freely modified, as are all FOSS programs.
And remember, you can't have freedom without "free" in the first place.