Introduced in January 1986, two years after the original Macintosh, the Plus came with 1MB of RAM, a new double-sided 800KB floppy drive, and a built-in SCSI port (the first Mac so equipped). Not only was 1MB more RAM than the PC-class machines could handle, but the Plus could be expanded to 4MB total RAM!
The original Plus came in Apple beige; during 1987 the case color was changed to platinum, the same color as current Macs. There are unsubstantiated rumors that the power supply in the beige Plus was less reliable and more prone to failure with 4MB configurations, a problem resolved with the "new" Plus. (All desktop Macs since the Plus have included a fan to reduce internal heat.)
The Plus was the last Mac not to use ADB ports for the keyboard and mouse; it was the first to use DIN-8 serial ports.
Featured in Bob's Computer Museum and Mac Plus Sites of Interest. Read the BYTE Product Preview. Here's one user's Mac Plus story.