How can you determine what version(s) of .NET are running on a system?

Question

What are the different ways (programmatically and otherwise) to determine what versions of .NET are running on a system?

Answer 1

Directly from the source:

How to determine which versions and service pack levels of the Microsoft .NET Framework are installed

Answer 2

If you're wanting the current framework version in use then you can see that via:

System.Environment.Version

Answer 3

I found How to check .NET Framework version installed much more usable. Essentially, open Internet Explorer, and paste this into the address bar:

javascript:alert(navigator.userAgent)

I don't know if it always works, or if it is complete, but it works for my uses, doesn't require a lot of extra reading, and works without installing anything additional.