I took my son to see Ninja Assassin back in Dec. The story is so-so but the action is awesome! It is a very fun movie.
Since then I wanted to check out other Ninja movies and have seen the following that are pretty good. Now, keep in mind, these are cheesy ninja movies basically a popcorn movie that is just plain entertaining.
Ninja (2009) This not the same as Ninja Assassin. Not too many people heard of this movie but it's pretty fun.
Azumi & Azumi 2 - Both are really good but Azumi is far better then the sequel. You can rent the first one on Netflix. I don't know why Netflix doesn't have the second one. I had to order it from an ebay seller.
Let me know if you guys have any other fantastic Ninja movies to add to the list.