SNES: PocketSNES or Snes9xOD, I think they are better or worse in different aspects.
Sega (Genesis/Mega Drive, I guess): Picodrive, I think that's the only one.
GBA: There's only one, gpSP
GB: Gambatte
NES: FCEU320 has the best compatibility, NesterOD has better sound quality to me and sometimes it feels smoother, but some games don't work.