Best post I've seen today: Ari Paparo talks about the differences between del.icio.us and Blink. Blink was Ari's startup during the bubble, which raised $13 million (!) to build an online bookmarking service, but didn't take off with users.
The only way any of us gets to be a successful entrepreneur is by learning from others' mistakes, yet a lot of business culture focuses around never admitting that errors are ever made. So kudos to Ari, not just for being brave enough to be self-critical, but for helping a lot of new aspiring entrepreneurs to succeed.
The only quibble I'd have is that Ari presents del.icio.us as having succeeded already. Josh and his team at del.icio.us have built a great app, but for as popular as they are with geeks, the hard work is to bring the concept of social bookmarking (or, if you prefer, a shared recollection tool) to a larger audience.
The best history of del.icio.us I've seen is David's post about its evolution. I'm only pointing to it so I get the old-school cred of saying "I liked it back when it was muxway."