todo.txt (todotxt.com) - open source with a simple text file format. Items have (priorities), +tags, and @locations. There is a CLI interface for desktops and apps for iOS & android. Syncing over dropbox is supported.
I chose todo.txt after trying all the others because I can get command line efficiency and Smartphone portability. Everything is open-source (both the smartphone apps as well) and easily extendable. I prefer Taskwarrior for a pure CLI tool, but no visibility on any other platform.