Locus Online's Cover Art Gallery
The alphabetical listing by illustrator contains cover graphics, artist credits, publication dates and publisher names. It even keeps an ongoing tally of the artists in the field with the most covers published in the given year (some familiar names there at the top...:)).
The database is not all-inclusive. I may be wrong, but I suspect the gallery is a representation of the books and magazines that LOCUS MAGAZINE receives and reviews, and that means that a lot of covers aren't shown. Another thing to remember is that the majority of the cover graphics link to Amazon.com, which is great because you can read about the book in question. However, what some folks may not know is that some of those graphics posted on Amazon are occasionally catalog comps, which are then finalized for publication via refinements of the cover illustration and/or typography. So sometimes you're not looking at the final version of a given cover. A minor quibble, but that doesn't diminish the fact that the gallery's a TREMENDOUS resource. If you're a fan or professional involved in the field of sf/fantasy art, please give Mark a shout and let him know his effort is appreciated. Happy New Year, Mark, and thanks for making the whole field more informed!