Maniac Mansion

"Maniac Mansion is a graphical adventure game originally released in 1987 by Lucasfilm Games (now known as LucasArts). Maniac Mansion has become known among gamers and programmers for its highly-acclaimed gameplay and its introduction of new ideas into gaming, including multiple possible endings, multiple user-selectable characters with significantly different abilities, and critical clues contained in numerous cut scenes. It was the game for which the SCUMM ("Script Creation Utility for Maniac Mansion") engine was created and named after, which went on to be used by LucasArts for ten more years to create 13 original titles. It is the first game to feature Chuck the Plant (who is found in the library). Some of the characters appear in other LucasArts adventure games, notably the sequel, Day of the Tentacle."

Source: Wikipedia, "Maniac_Mansion," available under the CC-BY-SA License.

Individual credits:
Ron Gilbert: design
Gary Winnick: design, art