Speaking of Pan paperbacks, a fellow calling himself Honeypot Designs has repurposed some original art (mostly by Sam Peffer) to create some cracking James Bond covers for titles which were never in Pans of that era:
He's also done some rather good Richard Chopping pastiches. Here's the one for Solo, the most recent Bond novel:
For Doctor Who fans of a certain age, the novelisations by Target Books are possibly even more fondly remembered than the actual programmes themselves (certainly, the special effects were always better), and many talented Whovians have busied themselves providing covers to more recent stories which never got the Target treatment. There's scores of them on the web, but here's a small selection. I especially like the way that some of them change the title of the original episode to something more 'exciting', just like Target used to do:
A final mention has to go to Timothy Anderson, who has created some bang-on pulp covers for Blade Runner and Star Wars, plus poster art for the original trilogy reimagined as spaghetti westerns. Here they all are (and if you want copies for your wall, all these and more are available at his Print Store).