RetroAchievements adds Xbox 360-style achievement hunting and leaderboards to thousands of classic games and romhacks. Uncommon games often have romhacks that work with RetroAchievements. Check Romhacking.net for patches you can apply with utilities like Floating IPS. I’ve already recommended Rockman 4 Minus Infinity but it also has RetroAchievements.
Dynamite Headdy delocalization by M.I.J.E.T. (RetroAchievements)
The localization we got here removed most of the dialogue, added American pop culture references, and made the annoying tower boss fight four times as long. This fan translation doesn’t do any of that. Go collect those secret bonus points.
Pulseman translation by M.I.J.E.T. (RetroAchievements)
My favorite game on Sega Channel had you build up static electricity and then discharge it to attack and move around. Most of the team that made it went on to make Pokémon Red and Blue.
Super Mario Land DX by toruzz (RetroAchievements)
Also Super Mario Land 2 DX by toruzz (RetroAchievements)
Super Mario Land is the first game I ever beat and a fine place to get started with romhacks. This turns Super Mario Land and its sequel into Game Boy Color games with some graphics redrawn for better visibility. The RetroAchievements add some nice challenges to keep things fresh.
Extra Mario Bros. by ATA (RetroAchievements)
This romhack gives Mario a dose of Metroid-like exploration with collecting keys and permanent upgrades to reach new places. It’s still Super Mario Bros., so the screen only scrolls to the right.
Umihara Kawase translation by satsu (RetroAchievements)
A new Umihara Kawase game just came out and I’m afraid to start it because I get hooked on these games for a long time. We’ve got a wedding to plan! The original game’s RetroAchievements keep track of every door and backpack you’ve reached, so try to get them all.
Source: RetroAchievements, Romhacking.net, and My Life in Gaming