flickr
flickr is annoyingly slick. I messed around for a few hours trying to learn enough php to parse the RSS feed or use their API or something to get a list of recent photos on the page here, then I realised that there's a page that just does all the work for you. You need to mess with javascript, but I can cope with that. Don't I look silly now?
Also, I can get (atom? rssd+? Who cares?) feeds of anyone's photostream, I can upload from my mobile via email from anywhere (leading to silly photos), I can just link to anything without having to explicitly share it... As has been said to me, these aren't necessarily very clever features from a technical point of view, but it's all about the workflow, innit? It's so easy to use.