Webster's Dictionary defines a 'game engine' as 'the word you've entered isn't in the dictionary,' but if it were to define it, it would probably explain that a game engine is used as the overall architecture to develop and run a game – it gives developers tools to create the disparate elements of a videogame and then pull them together to create a functioning whole. From the renderer to the physics system, sound architecture, scripting, AI and networking, game engines either natively power every aspect of a game, or they allow other specialised middleware to slot into the game's framework. In any case, game engines are the workhorses of modern videogame development.
The list comprises the top commercial engines, many of which come with a 'sandbox' mode when purchasing a game allowing one to add in architectural, geographic and urban content to visualize.
Take a look at the Top 10 Game Engines at IGN for a full run down.