Definitely anything by NG.Dev.Team, though I will admit I have not played any of there games.
This is why I will
be buying a Retro VGS wait a few years for emulation.
Zaku is an Atari Lynx shooter that is a
ripoff homage to Air Zonk.
There's a bazillion Mario
rom hacks fan games out there for the NES and SNES. Some of them even have reproduction carts you can buy.
I definitely need to try Extra Mario Bros. Has backtracking and a pretty crazy final boss.
Then there is Second Reality Project, know as the best Mario World rom hack
There's also some crazy awesome Intellivion homebrew, like this Donkey Kong remake with 2 extra characters and all 4 levels.
http://www.intellivision.us/intvgames/dkarcade/dkarcade.phpÓscar Toledo Gutiérrez has done a couple of really good MSX homebrew. I'm too lazy to find his stuff.