Pirates vs. Ninjas is an internet meme regarding conflict between pirates and ninjas. It includes debate over whether pirates or ninjas are better and who would win in a fight.
Robert Hamburger's Real Ultimate Power website was one of the first internet appearances of the meme, in March 2002. Then, a popular flash animation that was influenced by Real Ultimate Power entitled How to Kill a Mockingbird, a mockumentary about the book To Kill a Mockingbird, appeared in 2004, aiding in the online spread of the meme. The animation begins as a genuine description of the book, but quickly turns into a science-fiction fantasy, a significant part of which involves a battle of pirates and ninjas. Several competitive web sites and games based upon the ninjas vs. pirates theme later appeared, including PVNgame.com and a popular viral Facebook application with millions of users. The site ninjasandpirates.net allowed members to join either the ninja or pirate sides and compete for control of the site every week. The piratevninja.com website invited visitors to cast a vote for either the pirates or ninjas, and explain their reasons for the vote.
The website weebls-stuff.com makes a reference to this meme in their flash animation series Weebl and Bob where a character named "Chris the Ninja Pirate" holds both the characteristics of a ninja and a pirate thus creating an "ultimate warrior".
The website MLIA has frequent debates about pirates versus ninjas through posting.
The forum for the UK domain of MMORPG Travian hosted a bi-annual event, to commemerate Pirate day and Ninja day, starting on Pirate Day 2009. This included battles between two forum user groups and unlockable userbars. (Forums)
The webcomic The Adventures of Dr. McNinja was created by Chris Hastings. Issue 2 of this comic features a storyline based upon the war between Pirates and Ninjas. The theme then follows on through other comics on the site.
A webcomic entitled PiratesVzNinjas was setup by Sam Shaw in early 2008.
Antarctic Press has produced a series of comics titled Pirates Vs. Ninjas which have a pirate crew taking on a ninja clan for the weapons of ancient gods. This was followed by a sequel called Up The Ante, plus an annual and several one-shot specials like Global Harming, Debate in '08, and Ghost Pirates Vs. Ghost Ninjas.
A webcomic entitled chaospet created by Ryan Lake recently featured, on September 14 2009, a character called Captain Ninja - a pirate and a ninja rolled into one. 
Another webcomic, established early 2009 by 'The Owl,' features random skits as well as a canonical storyline about this meme.