It's not exactly gaming, but it was a rather nice find nonetheless at another local Goodwill.
The Yamaha PortaSound PSS-130, a purely synthesized portable keyboard with two-note polyphony released in 1987. The unit appears to be in pretty nice condition, sports a variety of patches, and accepts four C cells, which were conveniently already installed.
I already own a wavetable PortaSound PSS-12, but it's in disrepair. One key is broken off, and another doesn't work properly. In addition, one of the speakers is loose inside the keyboard.