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.
Unity makes collaboration for little independent game studios very easy.
Furthermore it saved my freelance life, because I could iterate so quickly through different prototyping-stages. And that's important for certain projects, above all for projects which will feature complete new stuff!
This engine is so useful, it has so many things about it that are just so explicitly good about it. 1) It is simple to use and learn but to accomplish something great is hard so don't give up trying something. 2) The Unity engine is very new compared to Source or Cry engine. But Unity will soon surpass these older engines with many of its upcoming updates.
I highly advise this for indie developers who would like to create a game or a simulation of sorts.