I first asked, "Is the iPhone 3G good for Geologizing?" back in 2008. I asked again in September 2009 and touched upon earthquake-related apps in November 2009. (That was mostly a link to a review of apps by Gary Robbins.)
A more centralized list of mobile geoscience apps, organized by specialty and platform (iOS and Android), is on SubSurfWiki, "an encyclopedia of subsurface science." About 50 apps were listed as of mid-2012. And today, Bristol University volcanologist Elspeth Robertson has posted her top five geology apps on the group blog Between a Rock and a Hard Place.
Many of these apps are for professionals. Amateurs may have different needs, and I'd like to hear some recommendations in the comments. It would be helpful to specify the platform (iPod, iPad etc.).