I have seen one 4-in-1 game cartridge my friend had that worked perfectly: It had Super Mario Bros 3, Shadow Warrior (aka Ninja Gaiden), Probotector and one more I can't remember... all of them worked, even when it was all in Japanese and we couldn't understand a word. Another cartridge had that 100 or so games, though the remainder was full of duplications of the same crappy tank game - the difference being slightly different game colors...
My sister has two GameBoy cartridges like these. I think the other had like 30 games, the other about the same amount.
The same problem could be seen here: I must REALLY congratulate the makers for fitting so many crappy little games into such a small space. =) I estimated one of the tank games in that cartridge must have taken only couple of kilobytes, including graphics, and yet it was a lot better than the tank game in the NES cartridge...
The cartridge even had one real gem: Something called "Sonic 5". I didn't knew Sega has already started making games for GameBoy, at the time they had just stopped making Dreamcast and just said they would make games for GB too... Also the game was part 5, the graphics were ripped from some other Sonic games - title taken from Sonic 3D Blast, with the number "5" really badly slapped on top of the logo, the music sucked raw eggs through very thin straw and the game was really unplayable... (Updatish: So now Sega has announced Sonic Advance, but it's probably better game than the warez people's ripoff ever was =)
Some pirates have, according to the rumors, even made a Pokémon platformer by taking some game and changing graphics. Numerous other weird GB warez versions have been reported to exist.