Grammar Police are toned down, wannabe Grammar Nazis. They are obviously inferior to the glorious Grammar Nazi race, and are incapable of correcting even the most obvious mistakes in grammar.
Grammar Police or Grammar Cops are found all over the internet, but they can be found particularly in the Youtube comments section. This is due to nobody wanting the feeble, incapable race of Police to correct their mistakes on forums. Rather, people want the competent, strong-willed race of Grammar Nazis to correct them. Grammar Police also have a website: http://grammarcops.wordpress.com/tag/origin/ ,whereas they bicker over the insignificant portions of Grammar.