On their new exploration section, Twitter blocks is available for viewing and use. The viz is in Flash and is supposed to allow you to explore your neighbors as well as your neighbors' neighbors. I think the higher up the blocks are, the more recent. It's kind of hard to say. Other than that, I'm actually not really sure what I'm looking at. I thought it might be because I'm not following that many people, but I viewed the blocks for the public timeline and still had trouble deciphering. Maybe others will have better luck.
Update: Michal posted on the feedback they've been getting on Twitter Blocks that's certainly worth reading:
So we get this a lot: "Beautiful! But useless!". We've heard it in response to most projects we've done over the past few years (one exception has been Oakland Crimespotting, whose stock yokel response is: "no way am I moving to Oakland!").
This kinda surprises me. I think their other projects are pretty useful and informative.