I dislike "genres." What the hell is "alternative" anyway?
I'll listen to anything, as long as it's good (with the exception of rap. Especially stuff that most people call "gangsta rap").
My favourite band is CKY (*looks at username, heh heh*). All three of their albums are excellent.
I'm also a great fan of Clutch (if anyone else has heard of them, you are my new best friend, haha), especially Blast Tyrant. Queens of the Stone Age are excellent. Josh Homme is an amazing frontman, and Songs for the Deaf = Win. I'm also loving Era Vulgaris.
In fact, anything Josh Homme touches turns to gold. Desert Sessions, Eagles of Death Metal, Mondo Generator, Kyuss... all amazing.
If you're into Death Metal, I urge you to take a gander at World Under Blood - http://myspace.com/worldunderblood - two immense songs. Foreign Objects are brilliant as well.
Other bands worthy of an honourable mention - Black Sabbath, The Cure, Daniel Lioneye, HIM, Iron Maiden, Kiss, Metallica, Muse, Placebo, Queen...
Sorry for the wall of text, but you just got me started on one of my favourite subjects - Music.
Ratatat, The Beach Boys, The Beatles, Between The Buried and Me, Rick Ross, Jay-Z, The Mars Volta, Every Time I Die, any cool theme song, whatever i can really just jam to. It changes every week.