I'm looking for a to-do application and I'm having a little trouble finding one that fits my requirements. Hive mind to the rescue!

• Needs to have some sort of task dependency feature
  • I don't really care how it's implemented, but it needs to be able to support more than just a simple sequential dependency. (Being able to support sublists as tasks and having lists support both sequential and parallel completion would be fine, for example.)
• Needs to support a smart list of things that can be done now based on those dependencies
  • That is, I want to be able to see a list that excludes things that depend on other things that haven't been done yet
• Free
• Needs to be usable on a Mac
  • Can be browser-based, native app, whatever, just not a Windows-only app

Any suggestions?

Ones I've looked at so far that (seem to) fail my requirements include:

• Toodledo
• Producteev
• Do.com
• Nirvana
• Wunderlist
• GQueues
• Astrid

If you think that I've made a mistake with one of these, let me know. I may have.