iTunes 8 has this feature called Genius. It can create a playlist containing songs that supposedly go well with each other based on a recognized "seed" song. Well, see for yourself:
I started with a track by B'z, a Japanese rock duo. So, Genius picked Gackt (ok), Anna Tsuchiya (good), L'Arc-en-Ciel (good), but Ayaka? Even worse, Caramell and Vengaboys? LOL.