Blob Wars, which resembles Mario, has become a serious hit with my 9-year-old brother and a number of friends, which makes me happy since it could very well become their GNU/Linux killer app. I came across it while looking for some Debian games on the request of that brother, who was then responsible for spreading its fame.
Also, Ceferino, a more favorite of mine, seems to also be catching up with Blob Wars with that same game-lover crew.