There are quite a few baseball games on the store these days (including one from the MLB itself), but Homerun Battle 3D focuses on the best part of the game: hitting home runs. Com2uS released the iPhone version last year (and it's great -- it was recommended to me by the producer of Street Fighter for iPhone), but the iPad version just got released about a week ago and it's excellent as well. You control bat placement in the strike zone and swing by tapping the screen; that's basically it. The simplicity of the game really makes the fun shine, and extras like gold balls and multiplier pitches (every once in a while) make you feel like a real home run hero.
The free version lets you hit a few balls around, but the full version allows for character customization, some online challenges, and even head-to-head multiplayer. In-app purchases kind of spread a shadow over the whole thing -- some of the customized gear and uniforms can run very expensive. That stuff is easy enough to avoid, though. The core gameplay is a lot of fun, and whenever you want a quick round of batting practice, it's there for you. Homerun Battle 3D is $4.99 on both the iPad and iPhone, with a trial available on the iPhone.