Twttr is a new mobile service that helps groups of friends bounce random thoughts around with SMS. When we showed it to Jason Goldman (product manager of Blogger) he called it "present tense blogging." That's a great way to describe it. It's fun to use because it strips social blogging down to it's essence and makes it immediate.
Jack Dorsey is one of Odeo's brightest stars so when he told us about this idea that has been haunting him for six years we had to listen. It's not even remotely related to audio but it's an awesome idea so we told him to go for it. Jack put this thing together very quickly but it took a few months to get a short code.
Anywhoo, there are two ways to use twttr: on the phone and on the web. The phone part is entirely text based like an adventure game. In addition to posting to 40404 you can "follow biz" "nudge jack" or "get noah" and other stuff. On the web you can browse through your timeline, post, and do lots of other stuff. I find myself switching between the two interfaces when it suits me. It's good stuff.