We've discussed the excellent stand-alone Gmail client Mailplane several times. While there remain a few holes here and there, I consider it a must-have application for anybody who needs to manage multiple Gmail accounts. After about half a year in beta, the developer Ruben Bakker has declared the latest Mailplane beta as ready for Leopard. In addition, he has announced that it will soon be released for sale at a price of $24.95. There will also be a "family option" for an additional $8 that allows users to install it on up to 5 machines in a single household.
Mailplane remains in beta "for a short while," and you can still request a beta invitation.