Userlist, which launches on Tuesday after a month of testing on selected partner sites, is now available on the instant community page - you can see it by hovering over the "buddies" section. You can also get your own by signing up on that page. The easiest way to explain Userlist is just by demoing it - I've embedded it below, for instance, although I doubt many people will be here on a Friday night. In short, users enter a chat name, and they can then see who else is on the site and click the names to interact with them. I still prefer the Webchat app, but I guess Userlist is useful as part of the whole package.
Useplane is serving much higher-level clients than us bloggers, and I'm sure they don't need my advice, but here's what I hoped for when I first saw Userlist: a cross between MeeboMe and MyBlogLog that shows all the Userplane users who are on your site, and lets you chat with them. No pop-up window, either - just a widget in the sidebar.
You must have JavaScript enabled to use Userplane Userlist. Click to view mashable%27s%20Community’s Userlist Detail. Userplane Userlist™
Disclosure: Userplane is a Mashable sponsor, sends us wads of hundred dollar bills in brown envelopes etc.