Klax (Digital Eclipse/Midway, 1999)
I shouldn't have been surprised that Midway released one of the most colorful arcade games on GBC. It turns out this is actually a solid way to play it, because it has time to ramp up the difficulty more slowly when it isn't out to eat quarters as quickly as possible. A good version of an old game.
Klustar (Rebellion/Infogrames, 1999)
It's Tetris from all sides. You control the little black square in the middle. Any piece that falls into you or one connected to you becomes attached, and any time you complete a 3x3 block those pieces disappear and any around them fall inwards. If a piece falls all the way to the other side of the screen, it turns white and becomes a barrier. It feels a little barebones to have been a full release, but it's still a very interesting idea.
Knockout Kings (Digital Eclipse/EA Sports, 1999)
You can't tell from the screenshot, but this whole game was flickering and the sound seemed like it was coming through an underwater tunnel. Probably platform bugs. Regardless, the same itself is an extremely basic boxing sim where the dominant strategy seems to be to mash punch as fast as possible and walk at the other guy. Not particularly fun.
Koguru Guruguru: Guruguru to Nakayoshi (Sting, 2001)
This was actually released as a Nintendo Power exclusive in Japan. That probably explains why it jumps so abruptly from talking to a cute bear-like creature about how you'll be great friends to going outside and immediately being asked to do a Pokemon battle used the small ghost-thing above. You can do a very strong special attack at the cost of a big hit to your own health, which I'm surprised hasn't been an option in more of these.
Konami GB Collection Vol. 1 (Konami, 2000)
Konami released four of these in Europe, with two on January 1st and the other 2 on May 1st of 2000. The first one had Castlevania, Gradius, Konami Racing, and Probotector. That last game is better known as Contra. You're looking at Konami Racing, which is somehow the most basic of all the top down racing games so far but still found room for horrible engine noises.
Konami GB Collection Vol. 2 (Konami, 2000)
The other January 1st release had Parodius, Block Game, Track and Field, and Frogger, and confusingly was released as the fourth volume instead of the second when this released in Japan in 1997 for Game Boy. Parodius is above. It's a very simple Galaga-ish game. Not much more to say about it than that.
Konami GB Collection Vol. 3 (Konami, 2000)
Meanwhile, the third game was actually the second on original release. It has Pop n' TwinBee, Mystical Ninja Starring Goemon, Motocross Maniacs, and Guttang Gottong. You're seeing Goemon above, which is just listed as "Starring Goemon" in the menu for some reason. This is definitely not the prettiest way to play the game, but you could do worse.
Konami GB Collection Vol. 4 (Konami, 2000)
And finally, the last collection, which was really the third in Japan. It came with Gradius II, Castlevania II, Yie Ar Kung Fu, and Antarctic Adventure, shown above. It's an endless runner-type game about a penguin who needs to collect flags and dodge holes in the ice. Not terribly interesting, but the better-known games on the rest of the cart are probably why anyone would've bought this.
Konchuu Fighters (Digital Kids, 2002)
Pokemon knockoff that uses bugs instead of pocket monsters. The opening cutscene is well done and gives you more context than Game Freak has ever bothered to, but I can't say that I've ever wished I could go out and catch a level 12 centipede or whatever. Seems like decent execution of an idea nobody really wanted.
Konchuu Hakase 2 (J-Wing, 1999)
Of course, I say no one wanted bug-mon, but here's another game that's precisely that. So either someone actually did want that, or devs just insisted on continuing to make it. It's a little hard to say because this game came from J-Wing right before their games started to massively drop in quality. I also just noticed that my character is transparent in the screenshot, and I'm not sure how that happened.
Golf Ou: The King of Golf
John Romero's Daikatana
Kakurenbou Battle Monster Tactics
Keitai Denju Telefang