Yes, it is because it’s open-source! And everyone could download it from the official site. There are versions for Windows, Mac, and Linux, and for download, you will need ~10 MB of disk space.
If you will play I recommend you have more than 1 GB of disk space because the game hame autosaves. So you could back up your word if you make mistake. And if you play the online game will autosave games on your disk.
If you don’t want to download from the official page you could add OpenTTD to your Steam library and play the game from Steam. But why when you could have the original?
Where do I install OpenTTD?
You could install it in any folder. I think that OpenTTD works like a portable version. When I was young I had a flash dish when I have the game so I could play on computers in school when we haven’t admin rights.
Can I contribute to OpenTTD?
As I wrote – OpenTTD is open source. You could view the repository on the Github page https://github.com/OpenTTD/OpenTTD when you could see Issues (reported bugs) and you could participate if your changes will fix some bugs you could send merge requests after that there are “nightly” versions that you could “alpha” isn’t full version but some bugs are fixed there.