Commodore 64 - The last time I tried turning it on, it displayed what resembles a test pattern, but wouldn't load any games. Seemed like maybe a chip died. This was a secondhand system, but it worked when I got it.
Xbox 360 - RRoD after about 2 years of occasional use.
Game Gear - I have 2 systems, both of which are secondhand. One doesn't display video; the other has major audio problems and washed out video. Sounds like it might be one or more dead capacitors. I picked up a couple of cap kits, but haven't gotten around to installing them yet. Apparently the GG has a higher than usual failure rate for its capacitors
NES - my original NES from 1992 hasn't died on me, but I did perform some maintenance to resolve a couple of issues it was developing. After 10 years of frequent use, I replaced the 72-pin connector to resolve some minor issues getting games to load. It is a simple repair that takes 15 minutes and costs about $10. After 20 years, I replaced all of the electrolytic capacitors to resolve an issue with wavy lines in the video output. The capacitors only cost about $5 or $6, but it took a lot of time to do the repair since I'm not very experienced with that sort of thing.