Kunoichi no Ichi is hardly the first ecchi manga centered around female ninjas, but it definitely has some terrific artwork and has a lot of fun with the ecchi scenes.
Trap alert: The MC (the one with white hair and bushy eyebrows) is actually a male, despite being drawn with feminine curves at times. Proceed with caution.
Coming soon: High School DxD manga adaptation
Kunoichi no Ichi (4 volumes, ongoing)