I keep thinking that I’ve drummed this point home, but perhaps not, so: it still amazes me that almost everything we do online has a timestamp (if not two), and yet services are so bad about exposing them, and especially about using them as a way to organise your stuff.
Flickr has a calendar view (which perhaps isn’t as obvious as it should be, but it’s there). Tumblr has dated archives, even if they only show you them at a crude resolution. Elsewhere, though? Barely anything. Sigh.