What are Non-GNU versions of terminal commands?

I thought GNU was the (only) supplier of essential tools whose open-source code powers Linux distributions. However while browsing unix.se, I saw some questions that explicitly state they are dealing with GNU, such as a question on the safety of parsing GNU

. This must imply that there are versions of

that are not provided by GNU, right? What are these versions, who make them and where can I find them?