Apple's unreleased Mac OS X 10.5 'Leopard' OS is available now for pre-order on Amazon - and has immediately shot to the top of the US sales charts.

It's a spectacle. It's rare to see a product that won't be released for four months become the biggest-selling software on the giant retail website.

In a nose-jab at Microsoft, Apple's preview status OS is selling better than Microsoft's Vista or Office 2007 suite - despite the difference in general marketshare statistics between the two operating systems.

The experience isn't repeated in the UK as Leopard isn't yet available for pre-order on Amazon's UK store, though customers are able to register their interest here.

Second biggest-seller is Microsoft's Office Home and Student 2007 edition, the cheaper iteration of the world's most widely-used office productivity suite.

Number three on the list? You guessed it - it's Leopard again, this time the Leopard Family Pack.

Better news for Microsoft in the fourth-place contender: Microsoft's Office Student and Teacher Edition 2004 takes that prize.

Windows again in fifth place, where the XP version of Adobe Photoshop Elements 5.0 is placed.

You can check the Amazon US best-selling software list (updated every hour) here.