I recently discovered TaskWarrior, which is purely CLI but quite feature-rich.
EDIT June 2015: Since I wrote this answer years ago, taskwarrior has developed in an awesome project with lots of features and integrations. Among my favourite there are:
- https://inthe.am/about (web interface)
- http://mirakel.azapps.de/index.html (android app)
More taskwarrior tools here.
