I'm new to GTD and just getting my system set up. However, I've been using a software tool for other things and I'm really liking how it fits into GTD.
xReminder Pro is a really simple Windows program. You define reminders, and it pops up a reminder. You can schedule recurring reminders or one-time. It also includes a stopwatch and countdown timer.
I'm using Outlook for my calendar and my action lists, but I hate, hate, hate having my calendar cluttered with stuff other than specific appointments. xReminder fixes that for me.
Here's how I'm using it in GTD:
There are certain habits I want to develop, such as cleaning off my desk and collecting all my in-items by around 4pm in the afternoon. It isn't critical, I certainly don't want an appointment set up on my calendar. Instead, I put a recurring reminder in xReminder. When it pops up at 4pm, I can choose to ignore it or choose to snooze it for another 10 minutes.
I'm also using it as a tickler. For example, I always forget about haloween costumes until it is too late for me to do anything more than throw something together. Rather than put a piece of paper in my tickler, I wrote a reminder that says "Add a Project and Next Action for Creating Halloween costumes". It gets it off my action lists and I can still choose to ignore it.
It also serves as a reminder for "routines". I have a timer set for 10AM daily that just says "Check In: Have you done your Morning Chores" and the notes section has a list of the morning chores I want to do daily.
Other obvious uses include birthdays and recurring reminders like "Update and Submit Time Card".
I use the countdown timer when I have something on my task list I DON'T want to do, I'll set a timer for 10 minutes and work on the offensive thing for just that long.
Just thought I'd share this tool with y'all.