All browsers (except for Safari so far) have started to update their browsers more frequently adding more and more new exciting web technologies all the time: a new browser war with mostly winners so far. All these browser vendors are releasing very early versions to the public in order to get as much feedback as possible. For my own reference - and yours too, sure - here's a list of these early versions.
Opera Next can be run at the same time as the regular OperaFirefox has an Aurora version (far future, unstable) and a beta version (more stable). You can run them alongside with the help of the ProfileManagerChrome has some Canary Builds which can be run alongside your regular Chrome.Internet Explorer Test Drive versions are released on a regular basis and can run next to IE9 (on Windows 7 only)