I know of a couple:

• Runtime Container: reviewing the container logs on a running app is humorous. The app kind of talks to itself. I can’t remember where the quotes come from.
• Studio Pro: The max number of errors shown on an app in Studio Pro is 1337
• m2ee-tools: has “yolo” mode. It tries to answer any prompts automatically. It can actually be a useful option despite the name

There are currently 2 easter eggs in Search (not really Mendix, but all our Community products like Docs, Forum). These have been in search from the very beginning (2 years), because well, I love building easter eggs. But… The beauty of easter eggs is that I am not going to reveal them myself, you'll have to look for it yourself. I might reveal one of them if I get sufficient feedback on the following question:

Who loves Comic Sans?

Edit 26-11-2019: Ok fine… If you type in the search "can I use comic sans please", you will be able to view any Mendix Community website (like Forum, Docs) in a…. different light. (P.S. This also works on Mendix.com)

I know of at least one :D 

There has been one in the domain model in earlier versions. “Thou shall not divorce”

Someone was a Python fan

unzip /mnt/c/Program Files/Mendix/7.16.0/runtime/bundles/com.mendix.appcontainer.jar -d /tmp
strings /tmp/com/mendix/m2ee/appcontainer/actions|grep treadle

→

One on't cross beams gone owt askew on treadle

I haven't found any yet :(