The Amiga had true multitasking for a 68K Machine and even ran MS-DOS and MacOS with emulators. Problem is they could only get game makers to write software for the Amiga. The business software, video editing, etc came too late. They still make Amiga systems with PowerPC chips now.
AROS is an open source of AmigaOS 3.1: http://aros.sourceforge.net/