Today Matt Brindley offers up Flare. Among other specializations, Flare changes the tray icon color to show new messages have arrived, lets you switch between open rooms using Ctrl + the room number, and notifies you of new messages with a preview.
Flare is only at version 0.4 so if you dig Flare be sure to let Matt know. Your support will encourage further development. Thanks for developing this Matt! [Signal vs. Noise]
When launching Campfire, the folks at 37signals said that they built it so you wouldn't have to have special software to chat.