This project in progress by Radical Brad looks really neat. It is a graphics co-processor (including an entire 6502-compatible CPU?) for the Commordore VIC-20 made in 7400 logic running at 25Mhz on a solderless breadboard (though I think he plans to create a PCB for it eventually).

Linus Åkesson (lft) converted an old electronic organ into a MIDI controller and 8-bit synthesizer using two 8-bit ATmega88 microcontrollers. The synthesizer sounds very NES-ish with its bright square+triangle+noise oscillators. lft is musician and has several videos of himself on youtube playing very nice renditions of various chiptunes (from memory) on his Chipophone.