Windows/Mac/Linux (all platforms with Java): GTD Free, a Java-based desktop app, is one of the most straight-forward implementations of the Getting Things Done organizational system you'll see, but it also serves as a great introduction for the GTD-curious. Five tabs put your tasks in a sequential flow, and using them helps reinforce GTD's basic tenets in your brain. Bang out action blurbs in Collect, detail them in Process, file them in already-labelled folders like Actions, Someday/Maybe, and Projects, and mark them off in Execute. It's definitely not a lightweight, paper-like system, but it's a fairly clean implementation if you want to try Getting Things Done as it was really intended. GTD Free is a free download, works wherever the Java platform does.