My first computer, which I assembled myself. Not necessarily the motherboard, since it was crappy, but particularly the AT mini tower with its distinctive design, including a turbo switch, dual 7-segment display for the clockspeed, and the lovely big grey tumbler-style plastic power switch. Also the Tseng Labs ET4000 video card that I had in that machine. Maybe also the Sound Galaxy card that I initially had in that machine, before I sold it to my cousin and replaced it with a GUS. (I believe I still have the GUS somewhere.) Everything else about that machine was replaceable, in hindsight.
Also, although I technically didn't throw it out, a Diamond Edge 3D (OEM) card, powered by an NVIDIA NV1 chip, complete with Sega Saturn controller and the additional backplate and ribbon cable with Sega Saturn ports. Years ago, a friend wanted it as a collector's piece and traded some stuff for it. I don't exactly remember what I got in return. Actually, good on him, since he knew even then how special that card was. My loss.