The BBC Microcomputer System, or BBC Micro, was a series of microcomputers and associated peripherals designed and built by the Acorn Computer company for the BBC Computer Literacy Project, operated by the British Broadcasting Corporation. Designed with an emphasis on education, it was notable for its ruggedness, expandability and the quality of its operating system.
While nine models were eventually produced with the BBC brand, the phrase "BBC Micro" is usually used colloquially to refer to the first six (Model A, B, B+64, B+128, Master 128, and Master Compact), excluding the Acorn Electron; subsequent BBC models are considered as part of Acorn's Archimedes series.
Horizon was one of the first emulators available for Macintosh. Despite this, it has fairly good compatibility with original BBC software, allowing you to play titles such as Elite without problems. The Mac OS X version adds an improved GUI and significantly improved sound to the original version. Check it out on Richard Bannister's site.