A while back, we released a web services API for Hiveminder. The Hiveminder API powers todo.pl as well as a host of other small tools.
If you're an iPhone or iPod Touch user, you can download iMinder 1.0.1 the App Store. Built on the Hiveminder API, iMinder lets you access your tasks using an intuitive iPhone interface and transparently keeps your iPhone in sync with Hiveminder. (It does a bunch of other cool stuff too.)
When you start up iMinder, you'll see something like this:
(More screenshots after the cut)