iOS/Mac/Web: Syncing and minimalist to-do lists certainly aren't hard to come by. iPhone, Mac and web app Silo adds the ability to work collaboratively on lists to mix.
Silo manages to combine the best features of a simple to-do list like Clear with the sharing options of something like the previously mentioned Orchestra. Silo is simple, and very clean, but manages to have enough functionality that it's still useful. The iPhone, Mac and web app are also always in sync, so when you're sharing and editing lists as a group it works well no matter what platform everyone is on.
Silo for iPhone ($1.99) [iTunes App Store]
Silo for Mac ($5.99) [Mac App Store]