Boycott Advance is a Nintendo Game boy Advance emulator. It was specifically designed to run homebrew software and non-commercial games, but it does support some commercial games. There are some known issues, though, and the developer claims that newer games will probably not run. I have personally tried two games with this emulator, and they worked flawlessly. I couldn't find any homebrew software to run, though, so I can't say how that works.
As far as game emulation goes, Boycott Advance proves to work just as well as any of the other emulators I tried from this developer. When you launch the application, you will be welcomed by an "open" screen where you need to find the ROM file that you want to play. After you do that, a new screen will pop up showing the game. From the Preferences window, you can change quite a bit about how the game is emulated. There are options that you can change including graphics, sound and controls. Some of them are emulation effect size, video resolution, aspect ratio, screen mode, frame rate display, emulation of sound, etc.
As you may know, the resolution of the Gameboy Advance was really small. Thus, the first screen that you see when the game starts playing is quite small as well. Thankfully, you can drag and drag to increase the size of the window. The larger you make it, the worse the graphics will look, though.
In short, Boycott Advance is a nice Gameboy Advance emulator with a lot of preferences that you can change and support for most, but not the newest, games.