Michael, what exactly do you need?
ToDo Matrix is good for task management, but doesn't do things like tasks linkages very well. It has "delegation" capabilities, but these are a bit limited if you really want collaborative task management.
I'd suggest that you look into something that's browser based. In particular, something that supports a mobile browser. That way, you can and your partners can all login from your mobile devices or desktop browsers.
If you have very simple needs, you might find that Google Documents could be a good tool.
"Don't Forget the Milk" is also popular (
www.dontforgetthemilk), although it doesn't really do team work very well.
Similarly, "Remember the Milk" (
Remember The Milk: Online to do list and task management). This one has some rudimentary sharing abilities.
Don't discount Google Calendar (which now has some task abilities). This has some surprisingly simple but powerful sharing capabilities. And it works well in a mobile browser.