Ufouria (NES)
Run around, gain powers that let you access new areas, like an awesome, cutesy Metroid. Hold down while in the air to stomp. The European ROM is in English.

Mr. Gimmick (NES)
Stage-based platformer where you charge stars over your head and throw them at enemies. You can ride safely on the bouncing stars as well as most enemies. Press Down to cycle through your three inventory items, and charge Up and B to use them. The hidden last stage and good ending require you get a hidden treasure in each level, but they require inhuman star-jumping ability. Get the Japanese ROM (Gimmick!), it uses a special sound chip, so the music is better. If the graphics are garbled, try a newer version of FCEU.

Magical Pop'n (SNES) (Patch)
Fast paced platformer about a hyperactive princess with a sword. Gameplay consists of "wander stage until you find the spell that lets you finish", crushing enemies underfoot all the while.

Super Demo World: The Legend Continues (SNES) (Patch)
This ROM hack is basically what Super Mario World should have been: actually challenging, with a lots of secret areas requiring THOUGHT (or dumb luck) to find. Once you get the speakerbox item, bring it to other levels for cool hints and insights into development.

Violinist of Hameln (SNES) (Patch)
You are an asshole musician who abuses his sidekick by picking up and throwing her at switches, enmies; also forcing her to dress in goofy costumes such as a monkey or atomic bomb. The translation patch starts out okay, but turns to shit partway through, with the Engrish getting thicker the further in you go.

Drill Dozer (GBA)
A girl, her team of thieves, and her robotic drilling suit.

Ninja Five-O (GBA)
Ninja Gaiden and Bionic Commando had a baby and it came out wearing sunglasses.

i know of more but i was feeling a bit lazy, maybe sometime i'll make a more comprehensive post in my livejournal