Chrome: If you've ever been on a site and wanted a more immediate form of communication than, say, comments can offer, Chrome extension Talkita lets you live chat with users that are currently viewing the same website.
After installing Talkita, a small chat window pops up on the site of your browser from which you can chat with other users on the site you're visiting. It's pretty neat if you want to live chat with, say, other Lifehacker fans as you're reading the articles. It's even more useful if you're browsing an online store (say, Newegg or something similar) and you can ask advice of other users right then and there.
Of course, Talkita's pretty young, so it doesn't have a huge userbase at the moment, but the more people that try out the extension, the more likely you'll find someone on a specific site to talk to. So, as with all community-centric plugins, recruit your friends! Right now you can login with Facebook, though Google and Twitter connect are coming soon, as are extensions for Firefox and other browsers.
Talkita is a free download, works wherever Chrome does.