I've been using Things from Cultured Code for quite a while, and like it alot. It's still in beta, and has had it's share of quirks. Lately it's been solid, and the folks at Cultured Code have recently released the iPhone counterpart to the Things app. All I can say is "Wow".It's perfect.
GTD systems are great, but one of the shortcomings that hinder other apps and processes is taking that system with you. Your "to-do" list(s) should be able to follow you wherever you go. Otherwise, they loose most of their value (that is, unless you only plan on getting things done at your computer). Being able to take my lists with me makes me consider how to GTD outside of work. GTD to my car? GTD with housework? Etcetera.The application itself lets you add to-dos to projects and set tags, due dates and notes to each item.
The interface, which is very Mac-like, taking alot from the iTunes GUI, automatically groups things that you need to see in categories such as "Today", "Next", "Someday", etc. I could add a task to some project due in a couple months, but I'm not notified of it until it becomes relevant... all based on how I set it up.The iPhone version is the same. Full featured as ever... just smaller. One of the neatest things about the iPhone version is it syncs with the desktop version over Wi-Fi. When you open Things on your iPhone (when connected to the same network as your Mac... and of course, your Mac is turned on), it quickly syncs and lets you know what it updated. This is great because during the course of a day, I'm most likely to need my Things updated more often than, say, sync of my iPhone galleries or a full iPhone backup. Makes updating my GTD lists quick and simple.
Try as I might I still can't seem to be able to buy Things for Mac, but I'm sure they'll release the final version very soon. You can, however, buy a copy of Things for your iPhone or iPod Touch through the App Store. Every GTD system is a bit different and one that works great for one might not be ideal for someone else. Do your research first, of course, and give Things a try.