Have a look at http://www.todotxt.com/
This set of tools starts from a simple txt file, the most versatile, industry-standard, non-proprietary file format that will never suffer from versioning problems or can become corrupted by program errors, and that can be stored and opened on any system imaginable.
TodoTXT is a free and open-source set of tools for Windows/*nix/MacOS operating systems as well the popular smartphone/tablet OS'es that can help you more easily edit and filter your task lists from the txt file. They recognize certain keywords and as such support contexts and projects as per GTD as well as priorities, and all support Dropbox for synchronizing your single todo.txt file across all your devices.
For example: I have a todo.txt file on my home computer in a Dropbox folder. On my work computer, I have Dropbox as well so the txt file is constantly synchronized. I use the command line interface tool (Todo.txt CLI) on both systems to filter tasks per context or project (though if you prefer more graphical tools, you could always open the txt file in your favorite text editor, or even find third-party tools that work on top of the txt file -- they are all listed on the TodoTXT website). On my Android phone, I have the TodoTXT app (fully supporting all features of the TodoTXT format, including GTD contexts and projects) that works together with the Dropbox app, so I always have my todo list in my pocket as well.