iOS: The popular Game Boy Advance emulator, GBA4iOS has been updated to version 2.0 and now works without requiring a jailbreak or any crazy workarounds. On top of GBA emulation, you'll also get support for Game Boy and Game Boy Colour.
Once you download GBA4iOS, you can customise the look of the app, install skins and install your ROMs. The app supports a web browser to find and download ROMs on your own, or you can drag them over through iTunes. There's also a Dropbox option for syncing, but I ran into error messages every time I tried to use it. From our tests, games seem to play pretty smoothly, but you'll probably have different results depending on which iOS device you have.
To install GBA4iOS, just head to the link below in mobile Safari, and click the download button. The app will download and install onto your phone or iPad from Safari.