Leo's pick on MacBreak Weekly 374 is Airmail for OS X by Bloop. This is a fast and modern alternative to Apple's Mail app. It should also be quite familiar to fans of Sparrow, the Gmail client that was bought and since neglected by Google.
Airmail is a clean and simple mail client that extends the Gmail experience across all of your email services. It makes searching through all of your mail fast and easy, and has quick filtering options. Airmail can group messages either by conversation or by subject. It even uses the same familiar shortcuts from the web-based version of Gmail.
Check out episode 374 of MacBreak Weekly for more about Airmail.