Every few months, I end up deciding if what current task manager I am using is right for me. I initially used Remember the Milk and then switched to Google Tasks before settling on NirvanaHQ. Now that I've been using NirvanaHQ, I've decided to look around at other task managers.
I tried Astrid when I first got my Android phone. One of my biggest annoyances with NirvanaHQ is that they do not have an Android app and their mobile web app is not great. I found Astrid to be very powerful but the interface wasn't great and sync didn't always work. When it did sync to their website, I wasn't a huge fan of their webapp. If you're only planning to use your task manager on your phone, then Astrid may be a good choice for you.
Being an avid Lifehacker reader for the last few years, I decided I would try Gina Trapini's Todo.txt. Since I was looking at multiple task managers, the idea of keeping your data with you was extremely appealing. However, using the command line all the time and the fact that you have to pay for the mobile apps was not ideal. I actually discovered that I am more often near a web browser than I am to a text editor.
Since I have an iPad and an Android phone, I was looking for something cross platform. Wunderlist seemed to be an appealing option and is visually the most stunning of all the apps. The web app, Android app and iPad app are all gorgeous. My one problem with Wunderlist was that I had been spoiled by NirvanaHQ. Wunderlist lacked a lot of the nice features of NirvanaHQ mainly repeating tasks, subtasks, tags and schedule dates. Schedule dates is probably the least important of them all. I'm going to keep my eye out on it for the moment and see if I want to switch.
My friend Stan Rosenthal suggested his followers to checkout Workflowy on his twitter feed. Workflowy is probably the most unique task manager on this list. Basically the concept is you get one gigantic unordered list which you can keep adding tasks to and subtask all you want. Each subtask, you can also share with people and let them finish projects. It's a really cool idea but I found it to be a little too chaotic for me to manage my tasks.
This is the more project based version of Wunderlist made by the same company. Wunderkit basically takes Wunderlist and adds areas which have notes and status updates. The cool thing is that you can share an area with a friend. I haven't really experimented with it too much yet but definitely let me know if you want an invite in the comments.