I have no idea just how big this intersection might be (in terms of sheer numbers, it has to be pretty big, I guess), but if you're a Yahoo! Mail user and a Safari 3 user, prepare to smile, because Yahoo! says that you can now get in on the "All-New Yahoo! Mail" fun. "All-New Yahoo! Mail" is apparently an AJAX-based, slick piece of webmail software built on the Oddpost engine -- I wouldn't really know all the features because Gmail has wrapped its simple-to-use tendrils completely around my life, from iPhone to all the different browsers I use.
But if you've been dedicated to Safari 3 since it came out, and chomping at the bit to use this "All-New Yahoo! Mail," now's your chance. Yahoo! does specifically say "Safari 3 on the Mac," but I'd assume it would work for the Windows version of Safari 3 as well -- if you have any problems, they'll probably fix it, and then they'll have to call it "The Newer All-New Yahoo! Mail." God forbid they come up with something newer than that.