Unity runs on Windows and Mac. There is also a Linux alpha version available.
There are 4 different payment plans for Unity:
According to EULA: Companies or incorporated entities that had a turnover in excess of US$100,000 in their last fiscal year must use Unity Plus (or a higher license); in excess of US$200,000 they must use Unity Pro (or Enterprise).
Download the Unity download assistant.
Run the assistant and choose the modules you want to download and install, such as Unity editor, MonoDevelop IDE, documentation, and desired platform build modules.
If you have an older version, you can update to the latest stable version.
If you want to install Unity without Unity download assistant, you can get the component installers from Unity 5.5.1 release notes.
It is often necessary to install multiple versions of Unity at the same time. To do so:
On Windows, change the default install directory to an empty folder that you have previously created such as
On Mac, the installer will always install to
/Applications/Unity. Rename this folder for your existing install (e.g. to
/Applications/Unity5.3.1f1) before running the installer for the different version.
You can hold Alt when launching Unity to force it to let you choose a project to open. Otherwise the last project loaded will attempt to load (if available) and it may prompt you to update a project you do not want updated.