There's a fun little battle simulator called Shoddy Battle where you can make your own team of Pokemon and try it out against others. Most legendary pokemon are banned though, due to having super high stats and are overpowered. It's fun if you're the competitive type or you just like battling with pokemon. It's not a big file too, just like a chat program. So give it a try if you're interested. It's free too. :3

Download link: http://shoddybattle.com/downloads
Website that helps a lot: http://www.smogon.com/

Word of warning though: You may lose A LOT! But it's fun to learn from each battle. X3