I know there's been a similar thread before but since i don't have exactly the same kind of music taste i'll do my own.

I've been listening to the same bands for quite a long time. I find like 1 new band every 2 months so i need some new music now. This month i found The Mars Volta which i REALLY like. A really good band. I've heard that Blood Brothers should be a good band too. They have been compared to The Mars Volta. Are they any good?
Other bands i like:
Led Zeppelin
Deep Purple
System of a down
Children of bodom
Metallica
Iron Maiden
Red Hot Chili Peppers
Guns n Roses
Black Sabbath
Jimi Hendrix
Rage Against The Machine
Dream Theater

That's some of the bands i listen to right now. So, do u have any suggestions of some new (or old) bands that i probably would like?

well if you like The Mars Volta, you surely have to check At The Drive-In out, as it's the previous band of the singer and the guitarplayer of TMV... check Relationship of Command...
...And You Will Know Us By The Trail Of Dead is also a very good band in that genre... Their most acclaimed album is probably Source, Tags & Codes...
Tool, maybe, check Aenima and/or Lateralus...
I don't know the Blood Brothers too well, so I can't really comment on them, the few things I heard of them seemed a little too hard for me though, well a year ago at least...
Oh and Mr. Bungle, Tomahawk, Fantômas and Faith No More might also be something for you...

ok. thanks orpheus.
well, i've allready checked out tool..quite a long time ago actually and i didnt really like them.
but ill check out the other bands. thx!

You may like Alice in Chains (grunge rock) or MC5 (late 60's proto-punk)

Yeah as Junkee said check out Alice In Chains. And so more grunge rock like Pearl Jam or Soundgarden you might like.

Blood Brothers are a good band but not too much like The Mars Volta in my opinion. If you want to check them out, get the album "Burn Piano Island Burn".

Blood Brothers(check out the the track Ambulance vs Ambulance) are intense and really energetic. Considering that you seem to like hardcore-ish stuff I'd also reccomend Coheed & Cambria(track; Delirium Trigger) and Between the Buried and Me and especially Codeseven(their album 'the rescue' is amazing and their new one is great too).

Considering your listing of:
Led Zeppelin
Deep Purple
Metallica
Guns n Roses
Black Sabbath
Jimi Hendrix
...you really should check out Soundgarden and Alice in Chains (probably not so much Pearl Jam, but they wouldn't hurt either). And while you're at it, check out some of the harder stuff by SMashing Pumpkins, which would be the album Siamese Dream.

As long as you are listening to hardcore, don't miss Converge's "Jane Doe" album and The Dillinger Escape Plan's "Calculating Infinity" or "Irony is a Dead Scene".

You also might enjoy Refused, get the album "The Shape of Punk to Come" immediately.

Also,being that you like Dream Theater, check out Opeth. Get just about any album by them, particularly "Blackwater Park" and "Still Life.

Led Zeppelin - maybe you could check out The Who or some of robert plants solo work. "sixty six to timbuktu" is a great album to start with.

Deep Purple - Never been much of a deep purple fan, but i guess you could listen to 'Blackmore's Night'. richie blackmore and his wife does some renaissance music. pretty cool. "Shadow of the Moon" is a good album.

System of a down - Great stuff. maybe tool?

Metallica - try some Slayer, Megadeth or The Misfits

Iron Maiden - not a fan. just try anything like helloween, dio or judas priest.

RHCP - tough one. you could go listen to some punk music or some funk. your choice.

Guns n Roses - i don't know. la guns?

Black Sabbath - just try some stuff like MC5 or some 'grunge' stuff

Jimi Hendrix - this is a tough one. just go listen to some beatles!

RATM - i guess there's no other good 'rap-metal' band. stay out of this genre

Dream Theater - not sure. sorry.