TkDo is a simple Todo Manager. It is open source and it is free, but don't call it freeware: The code is distributed under the terms of the 2-Clause BSD License. The program stores everything in a single SQLite database file. So far it is translated into english and german.
There are binaries for Linux and Windows available, all others can use the source. In that case you need the Tcl/Tk interpreter as well as the Tcl/Tk libraries TkPNG and SQLite3.
What it is not
TkDo is not a calendar. I've started coding TkDo because any tool I found was based on a deadline on which something has to be done, but I needed something that just handles priorities between all my Todos.
Depending on your computer system and its capabilities it might look like this:
The latest version is: 0.4