I recently got a beta account at ifttt.com (If This Then That), I've been playing around and it's a really nice idea made into a really nice web app.
ifttt lets you set up tasks, based on the public and authenticated APIs of lots of web services; Twitter, Flickr, Facebook, Google Reader - then, you can choose an action to happen.
This can be as simple as:
IF a new blog post appears on a site THEN send me an email.
Or as clever as the APIs will allow:
IF my friend posts a photo on Flickr THEN post a Tweet about it.
The possibilities are endless, but what really got me was the awesome UI design. It couldn't be any simpler, clearer or bolder. It's truly a joy to use, just because it's so simple - and not an app you'll be spending every day using either. Screengrab below. That's not zoomed-in - it's really that big.