Horizon is a BBC Model B emulator for Mac OS X.
Horizon (formerly MacBeebEm) 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.
Horizon was initially developed for classic MacOS and Windows by Chris Lam. While the source code for Horizon is not available, the toolkit used to develop the Macintosh port will be available in the longer term from my developer pages. Certain portions are available now, and more will be added in the future.