Free!'s opening is really awesome, the first few seconds of the opening get you paying attention to it. The ending isn't as cool, but it's still really great.

Openings:
xxxHOLiC's "19sai"
Tsuritama's "Tsurezure Monochrome"
Humanity has Declined's "Real World"
Mushishi's "The Sore Feet song"

Closing:
The Cat return's "Kaze ni Naru"
xxxHOLiC's "Reason"

I really like the song. I love it. Free!'s opening is really awesome,

I watched The Cat Returns a long time ago, but because it's been so long I don't remember the music...should rewatch it of find that song....

The openings for Soul Eater were pretty cool. My favorites are probably the second (Paper Moon by Tommy Heavenly6) and the fourth (Strength by Abingdon Boys School). My favorite opening ever is 'Colors of the Heart' by UVERworld. It was used as the opening for Blood+ for the first season, I believe. I never got around to watching that particular anime, but UVERworld has done openings for Bleach and D.Gray-Man as well. They're probably my favorite J-Rock band. Naruto had some pretty awesome openings and endings. The people picking out the songs for it did a pretty nice job. :3

I always thought the opening to Big O was awesome. Kind of a homage to Queen's Flash Gordon theme. I love Queen.