Deadline - simplest web calendar ever?
The Deadline UI is well-designed and not distracting, with edit and delete buttons readily accessible next to each item. Inspired by Twitter, to-do items have a 140 character cap to accomodate SMS support (which is forthcoming for Deadline, but not quite there yet). The 140 limitation doesn't have much practical effect, because date, time and location information can be submitted through brief natural language, no fancy syntax involved. There's an XML API, too, if you want to experiment with Deadline yourself.