Plurk is the only Twitter alternative I'm excited about. Why? Because the interface is fun to navigate, gives a better sense of time and community, and has plenty of visual queues for it's features ... and the people who are building it are obviously having a lot of fun. Their logo is a headless doglephant, and they call themselves the A-Team. Never underestimate the power of fun.
They say it's built to scale, but we'll see. Their growth chart this week looks like an insane hockey stick, but I suspect they're still under 10k users. My fingers are crossed. I'd love to see these guys succeed.
In the meantime, you're welcome to Plurk me.