The latest iteration of the Mac operating system, OS X 10.9 ("Mavericks") is available today (in theory). It's free. Consult "Software Update" under your menu if you're feeling lucky and want to join the other rabid Macaholics on the bleeding edge.
Me, I always wait a couple of weeks for the X.X.1 bug-fix release so others can do the gamma testing on my behalf.
PS: Apple has done this many times before, so (unlike Obamacare's websites) they know what kind of download volume to expect, and they're ready for it. Still, expect times to be slower than usual due to the traffic jam.