These are the ones I use, they are for the most part fully compatible with custom firmware and don't require 1.50.
- NesterJ for NES
- Snes9xtyl for SNES (there are loads of variations of this one, it can be hard to find just the right one)
- Masterboy for GB, GBC, and Master System
- gpSP for GBA (there are a few variations of this one as well, at least one of them works on 3.xx and 4.xx firmware)
- Picodrive for Genesis as well as Sega CD
And make sure you get version 1.50 of Picodrive. It was just recently released and has loads of code optimizations resulting in way better performance.