An open-source Android emulation project based on Libretro. Its focus is a straightforward setup, good Android integration, and a gamepad-friendly interface.
Links: Source repository | Play Store
Cross-platform, sophisticated frontend for the libretro API. Licensed GPLv3. It attempts to be small and lean while still having all the useful core features expected from an emulator. It is designed to be very portable and features a gamepad-centric and touchscreen UI. It also has a full-featured command-line interface.
Links: Source repository | Play Store | Website