Well, in SHPS, I put some eastereggs into the commandline part (which I am quite proud of :D), for example quotes (which might not even be runnable, as one token cannot have whitespaces), commands which output quotes, config options which transform all stdout into 1337, and so on :D

At work, I usually write creative comments into the source code, for example, my PowerShell template includes at the end:

###################################################################################################
######################### <º)))><,¸¸.·´¯`·.´¯`·.¸¸.·´¯`·.¸ So Long, and Thanks for All the Fish ###
###################################################################################################

In my HTML5 Deck of Cards project if you click all kings in-a-row, you get aces in the deck. If you click all aces in-a-row, well..you should try and see what happens yourself 😉

https://deck-of-cards.js.org

I didn't put any myself, but one of our deprecated app had a classic konami code easter egg. A T-rex was crossing your screen when entering the code.

The funny story with this was that someone printed a report (there wasn't any feature for printing, he only printed the page from the browser) and the t-rex printed on the page. My boss had some explaining to do and we finally set it to display only for a specific user.

In the new version of the app, it displays random GIFs picked by some dev when they resigned. It's now a tradition when you resign (in good term)

I've put ASCII art at the top of CSS files, as well as printed ASCII art into the JS console in case people were looking at the source.

And in the past I remember putting this on some sites: http://zurb.com/playground/jquery-raptorize

In one of my applications, an internal dashboard for a client, I have an ASCII shrug ¯\(ツ)\/¯ show up in some places when something unexpected happens. Luckily I don't think it's been triggered much if at all, because no one has mentioned spotting it yet :)