I'm currently addicted to this song, which incidentally, is the ending theme from the anime Suzumiya Haruhi no Yuutsu("The Melancholy of Haruhi Suzumiya"). The name if this song is Hare Hare Yukai ("Sunny Sunny Happiness").
I think the most addictive thing about this song is the very energetic dance. Apparently it's so popular that people at cosplay gatherings around the world have been holding mass dances to this song. Some crazy person even organised one in a street in Japan. Unfortunately, the police came while they were on their second round...
That is the power of Haruhiism.
Incidentally, I want to learn to play this song on the guitar: "God Knows". It's an insert song for Episode 12. Hirano Aya, Suzumiya Haruhi's seiyuu can really sing!
I decided to embed this video, even though it isn't the best rendition of the song, because Hirano-san does make a good effort on the guitar. Judging by how her singing isn't her usual standard here, and how she keeps looking like a left hand when she changes cords, exactly like a beginner, I doubt she actually knows how to play the guitar. She probably just picked it up for this song and the other insert song, "Lost My Music". For better singing, and to catch some of the best guitar playing by an anime character (not Haruhi though, Nagato Yuki was the lead guitarist when the song was played in the series), go watch the anime!!
I think another seiyuu I should credit is Goto Yuko, Asahina Mikuru's voice. It probably took a lot of effort to sing in such an off-tune manner... =p