Far too lazy to include stuff, so I'll just say it, I guess:
Almost all Naruto OPs, Almost all Bleach OPs, the first 3 Eureka Seven OPs, and the main Gantz OP <3
Countless others, but these are probably the main ones :/
Oh and Blood+ OPs 1 2 and 3 (show sucked though xP)

http://www.youtube.com/watch?v=BrLnzJPESwg

Gundam 00 S2 OP2.

It's mirrored, btw.

Oh yeah, and the Beelzebub theme (YES)

Robotics;Notes OP, Zwei - Junjou Spectra

http://www.youtube.com/watch?v=p1r-M2K2Dlk


BTOOOM! OP, Nano - No Pain, No Game

http://www.youtube.com/watch?v=RxoPDZUej6c
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I watch over 600 Animes, i have many more good Ops XD
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SID - V.I.P, Magi OP is also good >n<

Ohh i have that song on osu, http://osu.ppy.sh, osu is a music game full of anime OSTs. You guys should try it! It's really awesome And is elfen lied similar to Mirai nikki?

Watch at your own risk:

And.. (Animes: Say "I Love You" and Mirai Nikki.)

Well, not really. It's about a girl from an "alien" race which have invisible arms that can cut through anything. She's a murder machine, and before the anime starts is subject to inhumane experiments. She escapes, gains a dual-personality disorder in which her other self is like an innocent child.

It's sort of about her coming to see humanity as not all bad, whilst also dealing with the scientist people hunting her down. Very, very gory, even better than Mirai Nikki uncensored.

OT, some of my favourites are endings rather than intros:

(k-on's official ED doesn't seem to be on youtube, but this is pretty neat.)(Yes, #2 is indeed an ecchi anime)