I played many times in 501 and in 1000 (Thousand)
The variant of 1000 (Тысяча) which I played is described here:
http://card-games.com.ua/thousand.html
I only played the version of 1000 for 2 players (so without a widow but with a pack and with marriage changing the trump suit) and 501 for 3 or 4 players
501 is quite similar to 1000, the differences are described here:
http://igry.dljatebja.ru/Kartochnye_...7sot_odno.html
http://card-games.com.ua/five-hundred-and-one.html
To me 501 is as interesting as Преферанс, but with much easier rules. Although I played Преферанс several times I forgot the most of the rules by now.
I also played of course in Durak (which everybody knows how to play as opposed to 501 or Преферанс) and in Веришь-не веришь.