Midnight Mansion HD
Midnight Mansion HD is a huge retro styled platformer with 11 different themed mansions to explore. Each mansion offers dozens of nicely designed rooms with treasures and keys for you to collect (and enemies and traps to avoid). The detailed map is also pretty nice because it shows you all rooms at once (including keys, doors and enemies).
The game is available for Windows/Mac and the game is pretty big as already mentioned (The demo alone takes already around an hour to finish).