sonic the hedgehog, tails, knuckles the echidna, cyclops, mileena, and 20 more (marvel and 24 more) drawn by pierre_roussel

This notch is not present on licensed US Genesis cartridges. It IS present on Japanese cartridges, which also have a different shape. The Japanese version of the Mega Drive 1 has a mechanical locking mechanism to prevent removing cartridges when the system is powered on, which this notch is for. Unlicensed and aftermarket cartridges include the notch so they can fit on any system from any region.

Ecco the Dolphin

Altered Beast (Juuouki) (characters: Centurion, Alanis)

Rocket Knight Adventures (character: Sparkster)

Golden Axe (character: Gilius-Thunderhead)

Earthworm Jim

Gunstar Heroes (character: Gunstar Red)

The Sega (Mega) CD plugs into this edge connector. The bottom of the console's case has a removable plastic cover for it (pictured below).

This is the removable plastic cover for the Sega CD edge connector mentioned above.

The Mega Drive 1/Genesis 1 has a headphone jack on the front; the volume control at the top controls the volume of the headphone output. This is also the only stereo output on the system; the AV port on the back can only produce mono sound. Later hardware versions get rid of the headphone jack but allow stereo over the AV connector.

Volume control for the headphone jack (near Sonic)

Streets of Rage (Bare Knuckle) (character: Axel Stone)

Mortal Kombat (characters: Scorpion, Mileena) (the Genesis version was noted for preserving the blood found in the arcade version, which was removed from other home ports)

Beyond Oasis (The Story of Thor) (character: Ruo)

The Zilog logo, upside-down. The Genesis provides their Z80 CPU for playing music and sound effects, freeing up the main 68000 CPU to do the rest of the game logic (although some games did music on the 68000 as well). The Z80 is also used when putting the Genesis into Master System mode with the Power Base Converter or equivalent.

Super Hang-On

It isn't clear which hockey game this is, but the goalkeeper is wearing a Detroit Red Wings uniform, and the black uniform with #99 would be Wayne Gretzky, who played for the Los Angeles Kings for the entirety of the Genesis's lifetime

Comix Zone (character: Sketch Turner)

Landstalker (character: Nigel)

ToeJam & Earl in Panic on Funkotron (characters: ToeJam, Sharla)

Shinobi series (character: Joe Musashi)