You probably know Final Fight, Streets of Rage, and TMNT Turtles in Time. All three are "oldschool" beat'em ups, developed in the eighties and nineties, but the genre changed with the time.
The Game-Art-HQ Community started an art collaboration where 15 beat'em ups from 1988 till 2010 were illustrated by around 20 artists, there are also lots of informations about the beat'em up genre, the illustrated game and what it added to the genre.
The games are:
Bad Dudes Vs. Dragon Ninja, Double Dragon II: The Revenge, Golden Axe, Captain Commando, Streets of Rage, TMNT: Turtles in Time, TMNT: The Hyperstone Heist, Legend, Battletoads, Comix Zone, Panzer Bandit, Castle Crashers, Muramasa: The Demon Blade, Castlevania: Lords of Shadow, Tron: Evolution
Not every famous beat'em up is featured there yet, and i guess not everyone would count Castlevania Lords of Shadow as a beat'em up too.