In case you don't have a rot13 binary installed on your unix(-like) system, I can recommend the bsdgames Debian package: besides rot13/caesar/morse coders, it also comes with tiny terminal games like atc and boggle. On shared servers, monop is fun if you're both attached to the same screen session (`screen -x`). Or you could just use one of the many boring alternatives like https://rot13.com :-)
Username meta info: https://news.ycombinator.com/item?id=31460706
