Mailplane - wow, I definitely need to check that out. A Webkit-based specialized browser for Google Mail that can handle multiple accounts. I currently use multiple Prism instances for this, but a single program with switching capability would of course be much better. Besides, Prism is nice, but (thanks to Mozilla technology) is simply not a Mac application. And Fluid would be Mac-like, but unfortunately it can only handle one Google Mail account (since it does not separate cookies or KeyChain entries).