I've just been rendered speechless not by one, but by TWO sites that truly embrace the spirit of what's web 2.0-y:
What's so special about them? They paid attention to what works and what doesn't and made it a priority to follow those guidelines.
I'm sure there's plenty of posts out there regarding good ideas or user experience guidelines, but here are some of the things that came to mind:
*Make it easy to sign up, easy to run away: cost of entry is zero
*Make it friendly, usable, snappy and PLEASE don't crowd it
*Make it interact with what I already use. New services should not be islands, they should enhance my current "internets" experience.
RememberTheMilk is using as many services as it can (Google's calendar, gears, iGoogle, maps; OSX widgets, mobile access, email interaction...). It also uses well-proven concepts like tags, tag clouds, quick search, etc. I like it. A lot.
Imified just so happens to be a service that can be used with RememberTheMilk. It's a service (plays well with others), it's cheap (as in free and as in I-didn't-have-to-jump-through-hoops). In particular, I like the IM interface. You just send a message to create an account! Brilliant! It responds with a menu! Brilliant! It's very creepily reminiscent of a text browser and a telephone menu... "press 1 to do this, press 2 to do that...".
Fantastic. I'm so happy! Try it! Do it!