Other than the GTD methodology, the Pomodoro technique is my favorite productivity tool for actually getting stuff done. In short, it's a way to maximize the time where you're highly concentrated and productive and minimize both internal and external distractions. It helps you achieve this by time blocking (typically 25 min intervals) and a strict set of rules that help you keep focused. I've been using it for a couple of years now and it really complements GTD well.
I've mostly been using it for studying effectively, and then Pomodroido Pro (a really good Android app) have been my weapon of choice for tracking my "Pomodoros". However, now I do more of my studying on my Macbook Air, and I've also started using the Pomodoro technique for programming, general todos and so on. Therefore I looked around for the perfect Pomodoro app for Mac OSX. After a bit of searching I found Vitamin-R, a really well thought through application that is perfect for following the Pomodoro technique.
You can easily note down stuff that pops into your head when you're working, you can see how much time is remaining of your current pomodoro at all times, it can play white-noise sounds to help you concentrate, and everything from notifications to other behavior is highly customizable.
Considering the time it will save you by eliminating procrastination, the price tag of under $20 is a good investment and well worth it. However, you don't have to take my word for it, as they offer a 2 week free trial (no credit cards needed). So give it a shot. I also recommend that you read or at least skim through the free Pomodoro technique e-book before trying it out, so that you really understand how the Pomodoro technique can help you.