I started writing a comment to AnnieB's Teriyaki Boyz story, but quickly realised that the following links deserved a separate post.
First off, NIGO. I bought Nigo's (B)APE SOUNDS a while back, during a trip in Japan. I was in a Virgin Megastore in Tokyo and decided to buy a bucket load of Japanese music to take back home as fun souvenirs. I picked up CDs completely randomly and I actually ended up liking most of what I had bought. Not only that, but it also kicked off an interest in Japanese music that carries on to this day. I picked (B)APE SOUNDS off the racks for no paticular reason except that the CD cover looked really cool. I got lucky: it's a great album. The fact that I don't understand the lyrics, not being a Japanese speaker, is not really a problem to me (it could be a put off for some people I guess).
Here's a song from (B)APE SOUNDS, called "Planet of the Babes".
Now, here's another major hip-pop act from Japan that I like: M-Flo.
M-Flo is known to bring together foreign influences and Japanese sensibilities. Here's a really good example of that: M-Flo + Monkey Majik.