I'm rather partial to Firefox and Thunderbird, even if they are still "developer releases". Thunderbird (and Mozilla Mail by matter of sharing most of the same codebase) is not the best email app I've seen, but it's the most consistent across all my platforms (OS/2, Win2K and MacOS X). Mail (at least in OSX 10.2) looked/felt a lot like Thunderbird in fact, so again it wasn't the best mail app I've seen, but I haven't seen any better on Mac is the thing.
As for browsers, Safari is nice -- looks/works a lot like Firefox in fact, though arguably the fully native interface of both Mail and Safari make them more "at home" on the Mac. I've just grown accustomed over the years to the way Mozilla does things and I find Firefox to be fast, stable and quite compatible with any websites that don't specifically look for IE versus "Netscape" and ignore all others.