Lion might just be the final cat.
The initial Mac OS X was the first version of a stable, UNIX-based OS that succeeded Mac OS, which was used by Apple between 1984 and 2001. Over time, Mac OS X changed quite a bit - Apple moved the architecture from PowerPC to an x86 and upgraded it from 32 to 64-bit.
Mac OS X is based on a range of technologies developed by NeXT, a company that was founded by Apple co-founder Steve Jobs and bought by Apple in 1996, which was also the mark of Steve Jobs' return to Apple. Since its original launch, Apple introduced seven major versions, all of which were named after big cats (Cheetah, Puma, Jaguar, panther, Tiger, Leopard, Snow Leopard). Mac OS X 10.7, code-named Lion, was first shown at an Apple event in October 2010 and will be first to include support for the Mac App Store. Lion is currently available as a developer preview.
The PowerPC-Intel transition, announced in June 2005, was the biggest change in the operating system's history so far - a critical move that could have alienated Apple users as Apple loved to discredit the performance of Intel processors in PCs prior to the announcement. All Mac OS X version up until Leopard (10.6) support the PowerPC architecture. Snow Leopard was the first to exclusively run on x86 processors.