Game Console Cartridge Region Adapter - Dismantled for reference and curiosity
While not an 'official' SEGA release, or for that matter a release that anyone wanted to properly brand, NTSC to PAL / PAL to NTSC converters were popular enough with enthusiastic gamers to include here.
You can read online for the differences between NTSC and PAL and why they exist however for the purpose of the Mega Drive, the console needed to output differently depending on the region it was used in due to the bulk of the televisions it was going to be used on conforming to either the NTSC or PAL formats.
The Mega Drive was also built with a regional lockout feature that prevents most games not intended for release in that specific region from running.
Converters such as the one below sat between the console and the ROM cartridge and tricked the Mega Drive into allowing games not sold for that region to play; most commonly US Genesis cartridges. It also dealt with screen resolution issues that were inherent due to the nature of PAL vs. NTSC displays. Some NTSC games played noticeably faster on PAL systems and a good converter will compensate for that too.
For general reference the Mega Drive can output at the following resolutions:
[PAL] 256×240, 320×240, 256×480, 320×480
[PAL and NTSC] 256×224, 256×448, 320×224, 320×448
[PAL and NTSC SEGA Master System Emulation] 256×192