What are some good apps to download for an Android smartphone that support RPGs?
There's this 5E character generator for android.
https://play.google.com/store/apps/details?id=com.wgkammerer.testgui.basiccharactersheet.app
It's a WIP, but is pretty good so far.
The Crawler APP for DCC. Even if you don't play DCC, it has a spectacular dice-rolling feature.
The Crawler App for the DCC RPG is the most useful app for a tabletop RPG ever made. However it specific to the DCC RPG. Although it does have a good dice roller
Game specific: the RQ6 "Combat Effects" app.
Generic: "Dice Bag"
Honestly, though, I'd have to say it's rare for me to use either one. In almost all games, I have real dice and printed reference sheets. The most common use of a mobile device at the table, in my case, is the use of my Nexus 7 to show players images I've selected before-hand and have ready:
"The sea cave entrance looks like this..."
"The mummified witch looks like this..."
"The jungle-covered ruins look similar to this..."
That kind of thing.