Windows/Mac/Linux (All platforms): There are tonnes of great instant messaging applications available, but one nagging problem in the IM world is that there's no great, fully cross-platform chat app (unless you count Meebo). Free, open source application Instantbird aims to do for chat what Firefox does for the browser. Based on the same tools used to create Firefox and Thunderbird, Instantbird should ultimately be just as extensible as the former—which means IM customisation will be as simple as installing a Firefox extension. Right now Instantbird connects to virtually any IM network and supports a tabbed chat interface. Beyond that, the application is still very barebones, but its upcoming features are very exciting. Aside from the basics you'd expect from any chat app (like status notifications, contact management, and IM notifications), the ultimate roadmap for Instantbird includes video and voice support. What's most exciting, though, is the possibility that the chat application will be subject to the same community of creative extension developers that make Firefox such a killer browser—so that Instantbird can easily be customised to deliver exactly what you want from it. Instantbird is free, works with Windows, Mac, and Linux. It's still a very young app, so this one's for the early adopters.