Unity is a multiplatform game development tool, designed from the start to ease creation. A fully integrated professional application, Unity just happens to contain the most powerful engine this side of a million dollars.
A good quality engine that can offers a lot and the games show it. Unity truly is worth investing in and many many products have been made on it. Popular Steam products as well! If you want a easy but flexible engine to make a game with, then Unity is the right choice.