A nice annotated list here. Although those listed in the post are linked to the iOS versions they are also available from google app store when I checked . (We would not need #5 and #9 so did not check these.)
This is a really interesting annotated list of apps links from Greg O'Connor of Spectronics focused on literacy support. The apps come from the Australian app store.
This fascinating presentation by Benedict Evans shares some staggering charts, interesting factoids, and all manner of statistics about mobile learning, mobile browsing, and mobile everything.