My experience with Sparrow has been good in terms of the simplicity and the user interface. It makes your job of reading and replying to emails, simple.
Being an AIR app, GoAir and Live Quotes work on Windows, Mac and Linux. GoAir is an offline Gmail Client while Live Quotes shows you relevant information about company stocks.
The most important new feature added in Leopard is undoubtedly Time Machine, Apple's attempt to encourage the vast majority of users who never, ever routinely back up their data to change their ways. Time Machine automatically backs up a Mac's files to a separate hard drive (internal or external, though external is certainly safer and more convenient) or a network volume being shared by another Mac running Leopard.