[zest]

Not so experimental and creative:
 

Thee Michelle Gun Elephant:

 
The Blue Hearts:

 
I'm also a big fan of Shiina Ringo and Tokyo Jihen.

 

[Goku]

UVERWORLD..lolol thats all i no

 

[Maxvla]

Mainstream, but worth looking at:
 
Day After Tomorrow any album. It's all great stuff. In the vein of Aikawa Nanase, but a bit tamer.
 

 

[miapriyucot]

I recommend GReeeeN songs.

 

[Hakase]

Definitely going to support Nujabes, just got back from japan a few months ago and picked up the Love (sic) hexalogy and the instrumentals with it are amazing! 
 
I'll list a bunch of stuff:
 
Uplift Spice - jrock
Tricot - jrock
SiM - Jrock (I guess they're mainstream?)
KOHH - Jrap (is that a real genre name?)
Cyclaman - Metal
Foreground Eclipse - jrock (One of my personal favorites)
Asriel - Gothic Metal
 
I also second IOSYS but they are really diverse in their works/genres they dabble in.

 

[Lunatique]

Here are some non-mainstream Japanese musical artists that are on the electronic side, and all have very creative music videos: