There are three law dictionary apps available for download to your iPhone: the free Nolo’s Plain English Law Dictionary (reviewed here); the moderately-priced Barron’s Law Dictionary (reviewed here); and Black’s Law Dictionary, the most citable and definitive, but also the most expensive (reviewed here). All are available through iTunes.
For reviews of other useful iPhone apps, go to the University of Kansas Law School blog, where they have ranked the top 10 apps for law students, or check the iPhone J.D. web site’s index of apps.