Yarrr!
It really depends on what music you like, but here are some cool songs/bands
Liquid Tension Experiment does a series of instrumentals (Namely "When the Water Breaks," it's seventeen minutes long but it's really cool)
Mortiis is also an instrumentalist, his longest song that I know is "Emperor of a Dimension Unknown" And that's twenty-seven minutes long.
Demons and Wizards does a really cool instrumental called "Beyond the Ice"
Blind Guardian has a myriad of instrumentals, my favourites are "Weird Dreams," and "Gandalf's Rebirth"
I could go on forever but I'll spare ye the rest.
