Paso-kara-hoodai at www.music-eclub.com (Japanese online karaoke)

For serious Jpop Karaoke-ers, you can pay a monthly fee of 630 yen at http://www.music-eclub.com/pasokara/ and sing your favorite hits from Hikaru Utada, Crystal Kay, Double, Ayumi Hamasaki, Olivia Lufkin, Dreams Come True, Namie Amuro, etc. (Can't you tell I'm a girl? Yes, they do have guy singers, too, but I usually sing the girl songs.)

I also want to add that they do include American songs by Usher, Christina Milian, etc. You may look up by Japanese kana or by English alphabet.

Here it is by English alphabet:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Oh, and to play the songs, you need to install their MidRadio Player.