I was into a few when I was younger - mainly because I had an extremely screwed-up sleep schedule and laid in bed watching Cartoon Network immediately after Adult Swim ended, and there happened to be a lot of those kinds of shows on at those hours, but anyway!
Bakugan was my favorite - I'd always try to catch the show, and while I never actually played, I had a single Dragonoid ball that I opened and closed fidgetingly until it didn't work right anymore. :) Good times, good times...I also watched Yu-Gi-Oh 5Ds (although when I look back on that one, I remember the characters and story more than the battles/creatures), and an obscure one called Chaotic. (I need to watch some of these again sometime!)
I recently did some research into Digimon, and - I'm sorry, it seems like a perfectly good franchise, but I think I missed my window of opportunity to understand it. o.O So many types! (Not the virus/vaccine/data thing, the other type list that's like Dragon, Crustacean, Giant Bird, etc.) So many forms! Everything evolving into everything! (Pokemon is complicated too, of course, but I managed to get it in my head when I was a little kid with a more open memory, so...)