Pip is a dice-rolling simulator, a handy aid for any sort of game that uses dice--from Yahtzee to Trivial Pursuit to Dungeons & Dragons. Pip lets you roll traditional six-sided dice, along with rest of the holy hexad of polyhedral nerd dice: the d4, d8, d10, d12, and d20.
Pip emphasizes its elegant interface, foregoing the more complex functionality of other dice-rolling apps: you slide out a "tray" on the right side of your screen, then drag and drop your desired dice onto (or off of) a virtual black tabletop one by one. You roll by shaking your device, with semi-realistic (but weirdly low-gravity) physics, and you can "lock" a die by tapping it, so other dice can't move it. Pip also lets you save 10 groupings of dice on different screens that you can swipe through.
The best things about Pip are its convincing sound effects, dice collisions, and slick, simple, attractive interface, so look elsewhere if you want for more complicated dice features such as customizable formulas or more-exotic dice types. Unfortunately, Pip's price hasn't come down and its features haven't gotten any richer since its creation over a year ago--and there's definitely room for small but substantive improvements while still maintaining Pip's streamlined feel (for example, by allowing different colors for different dice of the same type, which would be a boon to RPG players). Pip is one of the better--and better-looking--dice-simulation apps, but check out the competition before you buy.