Amazon have listed Apple's upcoming OS X update, version 10.6 "Snow Leopard", for pre-order. As expected, the new software will arrive in September and be priced at $29 for an individual upgrade; however there's also a five-user "family pack" upgrade version for $49.
As the online retailer notes, Apple is yet to announce a specific date in September on which Snow Leopard will be released. More details as to what features and functionality the upgrade brings are available here.
Interestingly, in Amazon's "what do customers ultimately buy after viewing this item?" section, 50-percent of those looking at the single-license upgrade of OS X Snow Leopard then go on to buy a Windows 7 Home Premium upgrade for $119.99. 25-percent go on to buy the Windows 7 Professional upgrade, for $199.99.