About a week ago I noticed the todotxt-touch android application which uses a simple text file to store tasks (see http://todotxt.com/) and uses Dropbox to synchronize the file between the mobile phone and other computers.
I liked the idea of using a simple text file, but I couldn't find a good editor for this format so I decided to write one. Since I use both Linux and Windows I decided to use PySide (LGPL Qt bindings for Python).
Screenshot on Ubuntu 11.04:
Screenshot on Windows 7:
It's still in beta stages, but it's usable (I'm already using it), I will release a proper version with an installer a .deb package for Ubuntu very soon (I hope in a couple of days).
If you want to try it you can get the latest version from https://bitbucket.org/3david/qtodotxt
It's been a while since I did cross-platform desktop programming and I have to say I really enjoyed it, it took me a couple of days but once I got the hang of Qt and PySide I started to progress pretty fast.
I used Aptana Studio 3.0 as an IDE and I must say it has really matured since the last time I used it, the intellisense isn't as good as Visual Studio's, but that's just because Python is a dynamic language and as such it is very difficult to implement good intellisense.
Another thing I liked was the startup time is very quick (less than second on Windows and almost instantaneous on Ubuntu).
To install on Linux:
sudo apt-get install python-pyside)
bin/qtodotxt(you might need to run
chmod a+x qtodotxt)
To install on Windows:
bin/qtodotxt.pyw(you can create a shortcut for it)
I don't have a Mac to try it on, but it should be similar.