iOS/Web: In most cases, your to-do list is something you keep to yourself. Well takes a different approach, focusing on working with others to create collaborative lists.
Well isn't exactly a productivity-based to-do list; it's more along the lines of Google Schemer or Springpad in that it helps you keep track of ideas from friends. Well seems best suited for lists such as "Books to Read", "Things To Do on Holidays" or "Games to Play" -- lists you don't mind sharing publicly. Well allows people to follow your lists and add their own suggestions. You can make private lists as well, and then share them directly with friends. Well could be especially helpful for holiday planning and keeping track of recommendations. The app is available for free at the iTunes App Store or online.