Powder probably isn't the most polished roguelike RPG experience on the App Store (Sword of Fargoal is generally considered to be among the best), but what it lacks in graphical quality, it makes up for in ease-of-use. The genre is sometimes tough to get into, but Powder, originally designed for the Game Boy Advance, uses easy buttons and icons to let you guide your turn-based RPG character through random dungeons, killing monsters, gaining XP, and drinking weird potions of various colors, all while trying to avoid cursed gear. It's still not easy (roguelikes are notoriously ruthless affairs, and permadeath is still the rule), but the controls are surprisingly intuitive, and the included tutorial makes an often steep learning curve much more gentle, even if you're new to the genre.
Plus, Powder wins points from me for its flexible save state process; it's very easy to work your way through a dungeon floor, exit out to make a call or do something else on your iPhone, and then dive back in and pick up right where you left off. Best of all, it just recently went free; that makes sense, considering the other versions are free as well. If you still want to support programmer Jeff Lait, you can do so by donating on his Web site. Meanwhile, Powder is a free download in the App Store, and definitely worth checking out.