Remember the Milk is a free online task manager that, although not designed specifically as a GTD application, can work well as one. It’s extremely versatile and there are various ways it can be set up.
Your list of tasks are on the left. When you select a task its details are shown in a box on the right. You can add a due date, repeat and time estimation amongst others. You are also able to add keywords which is how I added contexts – @errands, @mac, etc. Notes can also be added.
There are five default lists or tabs when you first start – inbox, personal, study, work and sent. I used lists in place of projects and had my tasks within them. Generally this worked pretty well. A disadvantage is that when a project is completed I couldn’t find a way of archiving it. I think you just have to delete it.
To create a next actions list I marked actions as high priority and created a smart list that only searched for this priority. I don’t really like having to manually mark tasks as next actions but it works okay.