The Gladiator: Road of the Sword (IGS/2003) is an excellent, underappreciated arcade beat 'em up, that feels like a refined hybrid of Streets of Rage and King of the Dragons. Features polished pixel art visuals and a variety of combos.
Set in a classic Dynasty-era China, it's a hidden gem.
Crusher Makochan (クラッシャーマコちゃん) is an obscure game by CAVE on the 68000 hardware. This game is wacky and is hard to believe as the developers were mainly known for producing the best shmups in the business.
Dolphin Blue, the arcade by Sammy Atomiswave, is available for the Sega Dreamcast as a fan-made port. Developed by megavolt85, this conversion allows the run-and-gun "sea Metal Slug" game to play well on original Dreamcast hardware, particularly using GDEMU or similar solutions.
Seibu Kaihatsu’s Zero Team is a hidden gem of the brawler genre. Featuring four distinct heroes and incredible destructible environments, it delivers awesome combat with smooth animations and a vibrant comic-book aesthetic. A punch-heavy masterclass in arcade action!