The P2 theme is Wordpress's best answer to Twitter. It's pretty cool: http://ma.tt/2009/05/how-p2-changed-automattic/

Brad Fitzpatrick (LiveJournal founder) is at Google working on PubSubHubbub, which is a distributed push system for RSS. The Google Reader team is already experimenting with it (as a provider currently, not a consumer): http://googlereader.blogspot.com/2009/08/pubsubhubbub-suppor...