Swan.emu (WonderSwan Emulator)

Works really well.

EDIT: If you're playing on a Retroid Device or a device with a built in controller and can't get the controls to work in Vertical Mode for some games, there is a long fix. You have to set the "Console Options" in Swan.Emu to Vertical, rotate the screen in the Video settings to match the rotation side you want, then create a new controller profile (ex "Vertical") and remap the Joystick. Switch back as needed. I'll rate higher if someday it's automatic on game-changed orientation. Still a good EMU