Duotrope: Duotrope is a great website where you can search by genre, category, sub genre and more. You can search for magazines that accept both fiction & poetry, so this is good for people who write in both genres.
Selby's List: A great website that is focused on bringing you a list of experimental poetry magazines.
Every Writer's Resource: A list of the top 50 literary magazines.
About.com: About gives details about their top 10 small literary magazines!
Wikipedia: A giant list of literary magazines. Although unlike some of the other sites, Wikipedia doesn't offer you any other information, so you might have to do a lot of clicking to find a site that's right for you.
Web Del Sol: Web Del Sol's list of Literary Magazines includes both online and print magazines.
UPDATE: One of the lovely readers has pointed me to New Pages
New Pages is full of book reviews, awesome magazines and more. Check it out!