Roms Mania is described as 'A working online resource for roms'. There are more than 10 alternatives to Roms Mania, not only websites but also apps for Self-Hosted solutions. The best alternative is The ROM Depot, which is free. Other great sites and apps similar to Roms Mania are CoolROM (Free), KillerRoms (Free), The Old Computer (Paid) and CDRomance (Free).
EmuBox is a website that allows you to play retro games using online emulators (NES, Mega Drive/Genesis, Game Boy, SNES). This service has flexible game search, cloud storage, WebRTC multiplayer, achievements, statistics, friendlist, and much more.