There are hundreds of AAC apps, from very simple "soundboards" to the very powerful, sophisticated, and versatile apps like Proloquo2Go.

The simple soundboard apps are equivalent to devices like Big Mack, Step-by-Step, 7-Level, and GoTalk.

Verbally is a very good free text-based AAC app. Verbally Premium adds some features (smarter word prediction, phrase history, customizable phrases) for $99.99

Proloquo2Gowas the first, and is still the best, of the high-end AAC apps. Spokane Schools was an early adopter, back in 2009, and we now have more than 60 licenses in use. Users and Speech/Language therapists really like Sam Sennott's VocaSpaces vocabulary selection design, and the ease of customization. The version 2 upgrade
The Road to Proloquo2Go v2 (which should be available soon, as of April 25, 2012) will add new features and additional vocabulary selection methods especially useful for the K12 environment. Spokane Schools has VPP (Volume Purchase Program) licenses available for district and student-owned iDevices.

Recently several new companies have published apps that are in the same "high-end" class as Proloquo2Go (P2G), and have similar features and versatility:

Talk Tablet US is from the Gus Communication Devices company, which has made AAC software for various devices for 20 years. Talk Tablet US is cheaper than P2G ($89.99 vs $189.99), has more built-in icons than P2G, and has individually customizable button and page sizes/colors. You can also have several user setups, which currently is not an option in P2G.

Speak for Yourself works just like a Prentke-Romich Vantage or Vanguard (and the developers are currently being sued by Prentke-Romich). It costs $299.99. The vocabulary and selection method, like Unity on a Vantage, follows the Language AquisitionMotor Planning (LAMP) principles. Earlier screenshots showed that Speak for Yourself was actually using Prentke-Romich icons, but they have since switched to a different icon set.
I don't want to dwell too much on the lawsuit, but these two articles are very good for defining both the specifics and the wider issues:

SonoFlex is from Tobii, and it uses both core-based and category-based vocabulary selection strategies.

TouchChat uses Nancy Inman's WordPower vocabulary page selection strategies.