Online retro games? You mean like games playable via a browser or sites that host retro games?

Nearly all Spectrum games are playable via a java applet. checkout www.worldofspectrum.org

If you mean a place to download retro games, you could try retroremakes.com or perhaps you refer to abandonware? Dodgy dirty legal until they catch me abandonware?

Perhaps try the freeware section at games4win.com

Maybe you could be more specific...