Zenbe Lists, from downtown New York-based Zenbe is a must have to-do list app. It is heads and shoulders above any other app in this category. Competitors should be very, very afraid.
To do lists are important to mac users, because unlike Outlook's tasks, there is no "to-do" functionality built into the mac ical/macmail/mobileme suite. Even if you are an Outlook user, web based "to do" lists such as "Remember The Milk" are nice services for everyday use. I personally have been using RTM for over a year.
When iPhone 2.0 came out on July 7, 2008, a number of simple iPhone only based apps rose to fill the gap on the new platform. Some were paid apps (ex: iToDoList, $0.99), some were free (ex: To Do ), but none, up until Zenbe, combined both a great web-app, and a great iPhone app, into one. And, of course, the Zenbe app is free.
What exactly makes Zenbe so great? Well it starts with a great design — in both the web and iPhone versions. The minimal green interface lives up to the "zen" name, and generally makes you want to use the program. The feature set feels exactly right — including some elements (hierarchical lists, editable entries etc..) not found in other to-do lists, even the venerable RTM web app.
Generally, the app is very close to perfect. I am giving it a top rating, until I see another close contender in the category.