Apple Macintosh SE
I got this one for free from a colleague who had gotten it for cheap (maybe free as well) but did not have the space or time to make it function.
Some time prior, I had bought a collection of an external SCSI CD-ROM and external SCSI hard drive off eBay with the hope of using one or both with the Apple IIgs. Alas, some incompatibility prevented them from working. But they were originally used with a Macintosh, so I harvested the hard drive and put it inside this Macintosh while leaving both floppy drives in place. I also got a copy of System 6.0.8 from a guy online.
Having a working Apple IIgs really helped bootstrap my ability to get the SE working. Because I could already copy files to the GS via serial port and write to 800k floppy disks, I could download and transfer serial communications software to the SE.
It now has 3 boot partitions, each running a different version of the Mac OS. The primary is, of course, System 6, because it runs the fastest. I've also spent time installing most of the softare on this partition, so there's much less to do when using the other partitions, which contain System 7 and System 7.5.