.NET Reflector is a useful tool which allows you to peek inside of DLLs and see the code. It was written by a chap called Lutz Roeder and given to the .NET community for free. RedGate software bought it and left it free for years. To be fair to RedGate, the never said what would happen to the product. However, the community isn't happy having to pay for something that's been free for 10 years.
As you can see from the link, the wikipedia article was updated pretty quickly.