My wife and I both have iPhones, and we want to be able to share grocery and honey-do lists with each other. Most task list applications are not very well-suited to sharing lists, and most of the web solutions we tried were not much better. But then we came across Zenbe Lists
(iTunes link), which is an iPhone application that synchronizes with a web-based version. But even better, Zenbe Lists allows you to share your lists with other Zenbe Lists users.
Voila, my wife and I can share a few simple lists, and now when I'm on my way home from work and need to stop at the store, I can quickly check to see what we need and pick it up. Rather than a list pinned to the fridge that is never with you when you need it, it's right in my pocket (and her's) all the time.