Unity is a free to download cross-platform game development program designed by Unity Technologies. Unity provides multi-platform support to ensure that your developer experience as is flexible as possible. With so many options, you can spend much less time worrying about platform and much more time worrying about the quality of your project. Unity really is a developer’s dream.
The Unity Web Player is a browser-based platform, allowing you to take advantage of all the power of Unity, right in your browser. Unity Web Player also updates automatically, so you'll never have to worry about falling behind or using old software without realizing it.
Unity is the default software development kit for the Nintendo Wii U. Nintendo even includes a free copy of the software to accompany every Wii U developer's license. It's hard to beat support like that, especially in a video game industry this competitive.
What it all boils down to is this: If you're a developer looking to put out quality, memorable video games, then Unity is perfect for you. Between the sizable list of features, the vast array of available platforms, and the fact that it's completely free, Unity is everything you could want and then some.