The famous Minkoff's Battle System. Can use default battlers, or specialized animated battlers made specifically for this system (they're pretty easy to find). Does require some homework to figure out how to use it properly, but its highly versatile and very much worth the time it takes. Besides, it comes with instructions.
In case that URL doesn't work, just visit www.rmxpunlimited.net, go to RMXP at the top and click on Script Archive. On the next page, select Battle Systems. Minkoff's Animate Battlers should be the third one listed there.