There's also the Sega Neptune, a hybrid 32-bit console that played Mega Drive games as well as Mega Drive 32X games. Sort of the predecessor to to the Sega MultiMega/CDX which played MD cartridges as well as Mega CD titles. Not too unlike the Nintendo/Sony Play Station in that regard. Of course the difference here is that the MultiMega came out while the Neptune never did. People got to see a prototype/possible finalised version of the console at an event but it was quickly cancelled after because the Saturn was on the way.
Also, bringing the Play Station hybrid back up, I noticed that the CD's the disc drive uses are called Super-Discs to be more in line with the SNES/SFC. Too bad said CD-drive is busted and there's no Super-Disc games to try it out on. I bet the actual add-on version is still out there to be found too.
EDIT Typing this out also made me remember a few other consoles, and while they did release, they only did in Japan. The first is the Sony PSX, not to be confused with the PS1. An alternate PS2 that also worked as a DVR and general media device. It had the very first instance of the XMB, before the PSP. It also has the PS2 broadband device built into the console allowing for NetPlay and online features for select PS2 titles.
There's also the Panasonic Q. Born out of a deal between Nintendo and Panasonic this is a Gamecube that can play DVD's as well as other features. However due it's bigger size and shape it required it's own variant of the Game Boy Player as the regular variant is too small for one example.
Then finally there's the iQue Player, an N64 built into what looks like the baby of a Dreamcast and N64 controller. Released only in China by Nintendo and a company called iQue Ltd as a way to help curb the piracy in the region. The games for the console are stored on a 64MB flash card which gamers could take to iQue Depot which were kiosks that let you download and update the games stored on the card, not unlike how things worked for the Famicom Disk System add-on. The original N64 version of Animal Crossing is the last game to be released for the system.
And finally theres the 64DD add-on for the N64 but y'all know about that one.