I use the WordReference.com website on a daily basis, so when I heard about its iPhone app I didn't think it twice and installed it right away.
WordReference.com features all the tools you find on the website, that is, instantaneous translations from English into Spanish and vice versa, as well as monolingual dictionaries for both Spanish and English, and a special tool to help you conjugate Spanish verbs.
Each entry features not only the correspondent translation or definition, but also a recorded sample of how that word is pronounced, plus links to threads in the WordReference.com forums where that word is being discussed.
The WordReference.com app is intuitive and easy to use. You can also use it with your iPhone or iPod Touch turned sideways. On the downside, the Spanish conjugating tool looks too small on screen and is difficult to manage.
WordReference.com is a must-have for all students of Spanish: a comprehensive dictionary like the one you have online, right in your pocket.