The Game Boy Advance has many homebrew titles as well as ROM hacks/translation patches. The ROMs can be flashed to carts to be played on real hardware using the steps listed in Playing Game Backups.
There are hundreds, if not thousands, of titles and here are some of the most utilized websites for developers to post their work:
- GBADev (single download) - Homebrew focus
- Itch.io - Homebrew focus
- ROMHacking
- PDroms (alternate collection)