To the average Joe, none. But we historians here all know about the Milton Bradley Microvision, Entex Adventure Vision, and GCE Vectrex. But there were quite a few others, so obscure they aren't even memtioned in major classic video game books! Off the top of my head, I can think of the Colorvision. Looked a bit like an Adventure Vision, with like 5 games. I seem to remember Mini-Vid Breakfree as being able to accept different games. Anyoner remember this? Then there was the Entex Select-a-Game. And there were a few more. I remember one that looked like a Game Boy, but it may have been Japan-only. Come on historians, let's try and get the complete list!
Remember, not all of them count. It only counts if the handheld/tabletop accepts new media (such as cartridges).