Retro Ninjas!

[Exhuminator]

Ninja 5-0 / Ninja Cop on the GBA is very, very good.

Indeed. It's definitely in the top three 2D ninja games ever made.

[Nemoide]

How about THE NINJA for Sega Master System... aka Ninja Princess aka Sega Ninja.

(Actually since it's sort of an overhead shooter, I doubt it's what shwisha wants but it's still a fun ninja game! I mean, you're not just any ninja, you're THE Ninja!)

[marurun]

I third Shadow of the Ninja / Blue Shadows / Yami no Shigotonin Kage. The gameplay is pretty smooth (though the flicker and slowdown can be heinous, especially on the last level) but the music is what takes the cake. Natsume tunes at their finest.

https://www.youtube.com/watch?v=6uqQofm ... 8D2B2814E5

I also second Ninja Spirit. Despite being one of the TG-16 / PCE's best games, it is still quite under-appreciated among those who aren't die-hards. That's too bad, because it's furious, fast, and frikkin' hard. Also, the music is pretty darn awesome-tastic.

https://www.youtube.com/watch?v=GLMGRELPfDI

[Exhuminator]

Natsume's Return Of The Ninja for Game Boy Color is also rather decent.

[BoneSnapDeez]

How about THE NINJA for Sega Master System... aka Ninja Princess aka Sega Ninja.

(Actually since it's sort of an overhead shooter, I doubt it's what shwisha wants but it's still a fun ninja game! I mean, you're not just any ninja, you're THE Ninja!)

The ColecoVision homebrew of Ninja Princess looks amazing. Too bad it's so rare and expensive now.

On the topic of HG101, here's their "Ninjas!" section.
http://www.hardcoregaming101.net/themes.htm#ninjas

I especially partial to...


The Ninja Kids by Taito, a fun and goofy beat 'em up. Now available on the Taito compilations.

And now a super-obscure one I used to own...


Kyum Gai: To Be Ninja for the Tandy Color Computer. About the closest thing Tandy had to Ninja Gaiden.

[shwisha]

How about THE NINJA for Sega Master System... aka Ninja Princess aka Sega Ninja.

(Actually since it's sort of an overhead shooter, I doubt it's what shwisha wants but it's still a fun ninja game! I mean, you're not just any ninja, you're THE Ninja!)

Looks cool... I'll check it out.