For the most part, Sega Mega Drive games were distributed in plastic "clamshell" cases, however many, particularly in North America were released in cardboard boxes to keep manufacturing costs down.
Only one officially licensed game, Sonic & Knuckles, was distributed in a cardboard box for all regions, primarily due to its unusual cartridge shape. This list does not include special bundles.
Early Electronic Arts-published games were distributed in "hard-top" cardboard boxes in North America, much like PC games of the era. This is only applicable for games released in 1990 and early 1991 - EA moved to the clamshell design for its second batch of releases and kept this design for the rest of the console's lifespan:
Accolade did not acquire an official license from Sega for many years, so its initial set of games, originally distributed under the Ballistic brand, also used hard-top PC-style cardboard boxes. Some, such as Winter Challenge were re-released in clamshell designs in Europe. Ishido: The Way of Stones, Accolade's oldest Mega Drive game, used a flip-top design.
North America saw many of its later games (1994 onwards) released exclusively in cardboard "flip-top" style packaging.
Note: all Mega Hit Series games are also flip-tops.
The very common slide-box design was used almost exclusively for re-releases in North America, i.e. those previously distributed in clamshells.
Much like many of its early Sega Master System titles, Brazillian Sega distributor Tec Toy chose to package games in its unique style of cardboard packaging. Many were re-released in clamshell designs.