.su

In the past, there was a country called the Soviet Union, which included many countries such as Russia and Ukraine. The country stopped existing in 1991, but some people still wanted to use the old website addresses that ended in ".su" because it reminded them of the old country. So, they created a special place on the internet where they could use those addresses called the ".su domain." It's like a special little corner of the internet where people who like to remember the Soviet Union can still use their old website addresses.