I haven't tried these so I cannot comment on their quality. But this has a list. Of particular note is RetroArch, OpenEMU, and Gens as three FOSS options.
Edit: Also, alternativeto.net is usually a decent source for finding alternatives for specific software. Here's the list for Kega Fusion alternatives. This has some more options than the other link I provided.
Many people misunderstand. It's not that Orange Cats have a single braincell. It's that all orange cats on the planet share a single braincell among them. At any given moment only one orange cat may be using the braincell.