These are brilliant, here's some of the classics...
http://www.danmcpharlin.com/ and http://www.flickr.com/photos/danmcp/ (there's also a GREAT set of handmade cardboard synths to gawp at too).
(Note to self - sometimes I may lose sight of what is good artwork vs. artwork that's so cheesy and bad it actually comes full circle to being good again - I think there's a mix of it in this little collection but the Sci-Fi aesthetic is a bit of both I think) - anyway, I've had fun finding it - enjoy!
Massive collection of Sci-Fi covers - http://www.flickr.com/photos/emperordalek/collections/72157601339119906/
Philip K. Dick cover collection - http://www.flickr.com/photos/gojira2012/sets/72157606528643105/with/4560058197/
Wolfwing's got some good stuff of Flickr - http://www.flickr.com/photos/34204893@N08/
One of the best collections on Flickr - Dark Lord Disco's Cosmic Excerpts http://www.flickr.com/photos/darklorddisco/sets/72157618390464466/
Cosmic Noodles / Galactic Doodles - http://www.flickr.com/groups/1339583@N24/
I never get bored searching through Star Wars detritus (despite the prequels and all their disappointments) - you never know what you'll find when looking through Star Wars tagged pics as it's such a far reaching piece of pop culture. Here's the full Flickr gallery of some of the things I have found: http://www.flickr.com/photos/kathykavan/galleries/72157623704463457/ ...enjoy!