So...J-Rock and Pop

[The Summoner of Leviathan]

Utada Hikaru (though not the biggest fan of her latest English stuff)
Angela Aki (she is amazing)
Do As Infinity
Chihiro Onitsuka
Yuna Ito
L'Arc~en~Ciel
Stephanie

[*Devore*]

Flow
Utada Hikaru
L'arc en ciel
Asian Kung-Fu Generation
Home Made Kazoku

All of these guys are awesome, if you watch popular anime shows like Bleach, Naruto and FMA then you would have heard some of there songs.

[Shiny]

this?

Nah, it wasn't that. It was more like maybe a ooo ooo. I know this is not very helpful.

[Jessweeee♪]

Dir en grey

I've heard a lot about them over the years as well.

They're lovely n.n

My favorite Dir en grey song is probably karasu :3

[JKTrix]

this?

Nah, it wasn't that. It was more like maybe a ooo ooo. I know this is not very helpful.

YouTube - The 5, 6, 7, 8's - "Woo Hoo"

[Shiny]

Thank you, JKTrix! I like this song. It's so cheery!

[BarelySeeAtAll]

I've only heard a few songs of each, but looking at my artists:

Asian Kung-Fu Generation
Dir en Grey
Maximum The Hormone
Nightmare
Mika Nakashima

I think that's it, but I'm not even sure they're all J ish xD

[Momiji]

Hmmm, let's see.

Perfume
marble
ROUND TABLE
Maaya Sakamoto
May'n
Ayako Kawasumi
Supercell
Kenji Ootsuki
Bivattchee
Denki Groove
Little Non
Rie Tanaka
Alstroemeria Records
IOSYS

[JKTrix]

Morning MUsume. D:

Most of my exposure to the usual Japanese artists comes from anime. Most of the songs specifically mentioned in this thread are from anime.

As far as Gackt goes, I'm kinda diggin' how he's totally cool with the whole kids/nerds demographic. He's done a several songs recently specifically for anime and games (including a cover of the Gundam franchise's most legendary song for a major Gundam video game), he sung two songs for the last Kamen Rider show... in fact he's going to be 'acting' as the main character in an upcoming, high-profile anime.

I haven't really heard much of his stuff outside games/anime, though. I quite liked one of his most recent tracks, Setsugekka, which was used for Samurai Warriors 3. It also featured instrumentalists from other fairly big J-Rock bands like Nightmare and Duel Jewel, so it was a pretty big collaboration. He uses his voice in a way I hadn't heard him use it before.

I didn't really mean to make this post about Gackt :P he just has my respeck for being all into the otaku stuff.

I never really got much into VizKei, and most of the bands whose songs I liked are decidedly contemporary. I've gotten into folks like AKFG and Beat Crusaders (mentioned before), and others like Sukima Switch, Base Ball Bear, Sambomaster, Kimaguren, Bivattchee...

There are a few groups/artists that I do pay more attention to following, though. First is probably Ikimonogakari. When I first heard Hanabi come out of my speakers at the end of Bleach (I think it started the same episode that Beat Crusaders' "Tonight Tonight Tonight" did) I was stunned. Their singer has remained one of my favourite singing voices out of Japan since then.
One of my favourite voices overall is Maaya Sakamoto. Never really made it huge outside of the anime circle. She tends to do mostly 'easy listening' kinds of stuff when she sings, and she's quite the prolific actor--voice acting in anime and otherwise. She had a big concert just a few weeks ago, celebrating both her 30th birthday and her 15th anniversary of being in show business. I really wanted to go. Totally love her.

I'm a fan of http://www.youtube.com/watch?v=f9vfC4qnKzwPerfume[/URL], though that one is definitely less about the vocal performance, and more about the groove factor/entertaining dance moves. I can certainly tolerate them more than the groups like AKB48 and Morning Musume (too many people, too young).

[Shiny]

I've only heard a few songs of each, but looking at my artists:


Nightmare

I think that's it, but I'm not even sure they're all J ish xD

Ohhh, I forgot about them. I do like their song Varuna.

There is also Buck Tick which has a sort of gothic feel that Mana/Moix Dix Mois has.

[kotora]

Most of it sucks balls, but here are some good artists + cool songs:

Tokio (YouTube - [PV] TOKIO - Sorafune, YouTube - TOKIO - SeiSyun)
Asian Kung-Fu Generation (YouTube - Riraito)
Dragon-Ash (more like j-rap, but still awesome: YouTube - Dragon Ash - Shizukana Hibino Kaidan Wo, YouTube - Dragon Ash『Los lobos�)
SADS (YouTube - SADS - Boukyaku no sora PV)
Utada Hikaru (YouTube - Utada Hikaru - Exodus '04 (LIVE), YouTube - Utada Hikaru - Beautiful World)

[Laddy]

I'm a sucker for Visual Kei when it's not trying to sound like metal.

[Rye]

Have to second kotora's recommendation of Dragon Ash; that song from Battle Royale is my favorite, but all of their stuff is fantastic.

YouTube - dragon ash - shizuka na hibi no kaidan wo

[one_winged_angel]

Maximum the Hormone
Asian Kung-fu Generation (Seriously, did they do every anime opening/ending ever or what?)
exist†trace
High and Mighty Colour
Matenrou Opera
Onmyouza
Galneryus