I've mentioned Taskwarrior previously (command line task management, GTDable), but it's worth noting that it recently hit 2.0 and has an upcoming server component (in alpha):
I'm copying this from a previous comment of mine because I always end up writing about the great features of Taskwarrior:
Seconding taskwarrior for being a great todo handler. It has a minimalistic interface, but doesn't lack any features that you (might) find yourself wanting after a while of use. The IRC channel (#taskwarrior on Freenode) is active too. The command-line tools are perfect and let you make your system as minimal or as complex as you want. Over time, I've:
1) Set up a cron job that pumps my current list out to a text file behind an .htaccess'd directory. This way, I can see my list without needing SSH access.
2) I've also got a little Dashboard widget that pulls that text down, so I can swipe to the top-left hot corner and see them at a glance.
3) I'm using Alfred (http://www.alfredapp.com/) on my Mac, so I wrote a simple trigger called "task add" that connects to my box and adds it there. There's also a few posts out there for DropBox integration if you use that.
4) I didn't write this, but if you use oh-my-zsh, there's a plugin for Taskwarrior. I've learned about a couple options by pressing tab.
http://taskwarrior.org/