I've always been a sucker for platform games. I still remember playing Mario Bros for hours and hours. The concepts were so simple - run through the level, jump on the bad guys, don't fall in the holes.
Apparently I'm not alone in my love for platform games. Gamasutra has just published a 32 page feature covering the top-selling platform games of today and the past - A Detailed Cross-Examination of Yesterday and Today's Best-Selling Platform Games.
If you're interested in writing your own platform game, or just learning what makes the best platform games so much fun, I encourage you to spend some time reading this article.