Recs.
Updated
Pixel Dungeon is a traditional roguelike that is completely free, with pixel-art graphics and simple interface. The gameplay revolves around exploring dungeons to collect items/weapons to fight monsters that get stronger as you progress to new levels.
Note: Although Google Play store says that the game offers in-app purchases, it's only an in-app donation to thank the developer, it does not unlock or add anything. The game is completely free and open source.
SpecsUpdate
Pros
Pro Maps and items are randomized allowing for lots of replayability
In a true roguelike fashion, levels are randomly generated each time a new game is started. And Pixel Dungeon goes one step further by randomizing effects of all items as well so it's impossible to tell if a dropped item will have a positive or a negative effect based on its sprite.
Pro Great variation in items
Next to the typical selection of buffs, de-buffs, potions (that can be either consumed or thrown), weapons and armor (that have to identified and can carry with them bonuses and curses), there are also less common items like random teleportation scrolls, map reveals, etc.