Self-plug, I'm working on one, local, native, no web based ui, and minimalist. And it has no external dependency. The data is stored as simple text file, and the format is easily searchable with *nix tools.

Not going to be open source or free though, with 2 year perpetual license. I wonder how much that would interest you? My target is people who wants todo.txt like simplicity, but few useful bits. Only for Linux and Windows for now.