Google Wave, Google’s new communication and collaboration tool, allows you to open multiple waves (like conversations in Gmail or messages any email client) by holding the Command key (Control on Windows) when selecting subsequent waves.
Unfortunately, it appears you can only open waves in the vertical space on the right, which means anything more than two, maybe three waves at most, becomes untenable. It’d be great to be able open waves in their own window like Gmail messages or simple have more sizing flexibility, especially on wide or large displays.