Gmail notifier works only on recent Windowses, of course. I'm sure someone's written free analogues, but it's not like it would kill google to show a little love to other operating systems.
I expect Google will, given that their entire server farm runs on stripped down *nix blades. But ya gotta hit the masses first -- sad but true.
Gmail status works for macs--it won't tell you *who*'s written, but it will tell you when new email arrives with a tidy little menubar icon.
If you just leave a browser window open in Gmail, it updates your new mail count live, so you get a little number indicator -- not as noteworthy as a real notification application, but it works. I usually leave the window open all day on whatever computer I'm working at. Don't tell my employer.