I've been playing with tumblr, the lite-weight blogging service. I haven't really scratched the surface yet, but I'm sufficiently impressed to add links from my RSS tutorial.
Apart from the obvious (blogging), another use which has occurred to me is as a simple feed aggregator to people who don't want to get involved with more complex services such as Yahoo Pipes. Simply import the feeds you are interested in, then point at the RSS. Nice and simple. Anyone else like tumblr?