I developed a GTD companion iPhone application
Before the iPhone, I developed a web based GTD system for myself -- it had most of the normal stuff you see out there (e.g. context based to-do's, basic projects, a "someday" file). I had one thing that I found rather useful though, and I haven't seen it an any of the other systems I tried out.
Often, you have things that are like recurring events, but not really. They don't have to be done on a set schedule, and it's ok to skip them once in a while. I call these things Habits, and I developed a simple iPhone app that you can use in addition to your to-do app (I use Things, by the way).
You can learn more about my
GTD companion iPhone App on my site.
Basically, you enter in the habit you want to form and an idea of how often you want to do them (every weekend, every 2 to 3 days, every other day, etc). When you start up Habits, you get a sorted list of the habits you are supposed to do (overdue at top, time to do next, and then the upcoming ones -- which you can still do if you want).
Over time, you can see how good you've been. Right now, I show a simple breakdown of on-time vs. late, but I will be adding more on this in future updates.