for a start I'll recommend some Japanese bands (some aren't really indie, just non-mainstream):
check them out on Youtube or Google!

BUGY CRAXONE - Post-grunge rock, no holds barred, aggressive music. Love their kickass vocalist Yukiko, her voice has that raw feel to it, plus she's hot too . Some of their songs are absolutely head-bang worthy, others are just so catchy you won't know why. They did sign with a record label in their early days, but decided to go full indie and produce their own albums.

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FREENOTE - I'm not sure why they haven't made their major debut yet, their songs can certainly match, if not better, some of the music being played in Japanese radio in recent years. They've progressed from a punk-rock band to a more melodic and mellow, piano-oriented band (maybe because their guitarist left). The vocalist Chikako has a very soothing voice, and she plays the piano in most of their songs.

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THE BAWDIES - They're not exactly indie, they're currently signed under Victor Entertainment. But what sets this band apart from all the other bands out there is their style of music - old school rock & roll, straight out from the Elvis era, and all songs are sung in English! If you love rock & roll, you'll love The Bawdies.

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I have tonnes more on my playlist, but i'll save 'em for later. Do share more bands that you love!

For me,it'll be

X Japan

-Widely dubbed as the pioneers of visual kei,this band is simply rockin'.For me their songs sometimes border on metal but still considered rock.Some nice tunes are Rusty Nail,Kurenai,Weekend.And this band is hugely,hugely famous in Japan.


Maximum the Hormone

-Some people already know this band from their anime songs,What's up,people?! and Zetsubou Billy,which was used for the opening and ending theme of Death Note respectively.Metal band but somehow still has catchy choruses.

Polysics
- Their sound is something different and its not something that you often hear on the mainstream stage. Their style is like punk/pop/rock/synth/techo with the use of electro voices and stuff. Their quite popular and according to Wiki, have released an album in the United States and got rave reviews and great sales.
If you want to try out something different, wacky, crazy, fun, colorful, etc then you should check them out.

Polysics - Electric Surfin' Go Go
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Base Ball Bear
- Their style is like indie and British rock with a male lead singer, the female bassist does the backup vocals (though she does get to sing a few verses sometimes). Their songs are usually about teenage life and memories and most of the time has this feel-good sound.

Base Ball Bear - Changes
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^ ah I love BBB! and Changes is one of my favourite songs! their bassist Shiori is hot :D

UVERworld

Pendulum
They are a six-piece DnB (Drum n Bass) band based in England (originally from Australia). The vocalist (Rob Swire) uses a ztar (a guitar shaped keytar/synth). Their music is a mix of DnB, rock, electronica, and hmm some funk i guess.TRIVIA: They use 13 computers when they perform. :D They're worth a try! Their songs are very addicting, especially their song Witchcraft. That's it. Enjoy guys! :D

Alice Nine
Shou(vocal),Nao(drum),Saga(Bass),Tora and Hiroto(guitar)
their style are nice,like bishonen,and i think Shou have an angel sound
and the leader is Nao,i think Nao have a great skill on playing drum

check this PV
Alice Nine - Cross Game
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Dir en Grey
Once a Visual-Kei Band in their starting at 1997, but in times they left their VK look to be more ordinary. Though their main genre is Rock, their style are changing in every album they release. Thus no one can really define their genre. But however, they're one of few bands that can break the border of language into international level.


Their current formation which has never changed even once
Kyo (Vo), Kaoru (Gu), Die (Gu), Toshiya (Ba), Shinya (Dr)

LM.C
Their style is more like punk rock.
Check this PV:
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NICO Touches The Walls
This is one of the best rock band I ever heard in my life. They are formed in 2004, they signed to Sony's Ki/oon Records during 2007. I believe some of you guys know this band through animes like Naruto ,FullMetal Alchemist & [C].

NICO Touches The Walls - DIver
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NICO Touches The Walls - Hologram
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NICO Touches The Walls - マトリョーシカ
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Enjoy!
.clap.

Rather than posting the well known stuff, I'll post the lesser known stuff so people can discover new music.

UNCHAIN
Signed under Avex Network. I've noticed that Avex Network have a massive amount of amazing artists.
Probably most known for Super Collider:

Awesome song, and the bassline is highly fun to play, but pretty hard to figure out by ear.

Envy & Other Sins
Originally discovered these guys because they were on a show in the UK called 'Orange Unsigned Act' and I wanted them to win from the beginning (And they did, which got them signed) Although sadly, they've split up now.


OGRE YOU ASSHOLE
Their style of music is an acquired taste, but one that I enjoy thoroughly. I've yet to come across another person that actually enjoys their music, hence why they're not so well-known.


E.via
YES, YES, YES, YES, YES.
Greatest female rapper ever to walk the earth, also the cutest female rapper as well. She's actually quite well known in the world of speed-rap, and became well known in Korea for having a couple of songs banned due to language.
One key thing to note, is her versatility. She does quite the variety of songs, as can be seen below: