So, I thought I might share a few of my favorites with you guys. ♥
If you don't have a Nico Nico Douga account, it's easy to get one. Follow the instructions here.
【ニコニコ動画】APH/ぴ/っ/た/ん The Hetalia Pittan.This one is so cute! I could watch for hours.
【ニコニコ動画】【腐向け】APHでニゲルゾ☆ハイテンション: Very well done. It must have taken someone a lot of time and hard work to make this.
【ニコニコ動画】【某国擬人化漫画】パ、ス、タ☆だいおう（完成＋オマケ）: A two-in-one MAD. For one click of the mouse, you get an amazing Azumanga Daioh opening MAD, and a Caramell Dansen afterwards.
【ニコニコ動画】【ＡＰＨて/が/き】ヘ/タ●ヘ/タ/ユ/●/イ【完△成】 The Heta Heta Yukai. America, you drama queen, Haruhi's part suits you so well.
【ニコニコ動画】な/んか/の普/憫 I recommend watching this one alone... there's nothing really dirty
I should also mention that they use the characters' name names on Nico Nico, as in England = Arther, America = Alfred, Japan = Honda, etc. Just a heads up.