If you're unfamiliar, Ta-Da List is the super simple (and just as useful), free list manager from 37signals. Now, folks who visit with their iPhones will automatically be re-directed to an optimized version of the site.
I just logged into my own account, and it's fantastic. The pages have been built to fit so well on the iPhone's screen that you forget you're on the web - it feels like a stand alone application on the iPhone. If you're a GTD freak (and even if you're not), you ought to check this out. I tip my hat to 37signals. Ta-Da List for iPhone is great.