Important Note: Cover art is copyright-protected. Unless you own the art, always obtain permission to use cover art images, especially for any items you intend to resell, like stuff from CafePress.com.
1. If you've got a one-word or short novel or series title, have it embroidered on a baseball cap like this one: Most major malls have a kiosk embroidery service who can do them for under $10 each; you might get them a bit cheaper if you order in bulk from a logo shop (various prices).
2. Blogger will let you add your cover art to your profile page, which also adds it as an icon whenever you make a signed-in comment and on your blog sidebar under "About Me" (free, max image file size 50K).
3. I know someone is going to want this: How to make bookmarks using tables in Word. For people like me, try How to Make a Duct Tape Bookmark (free).
4. Create digital stickers of your cover art in a variety of sizes at places like 123Stickers.com. Stickers can be applied to anything (various prices).
5. Flickr has a neat a magazine cover generator that allows you to custom-design a magazine-style cover. Use your cover art as the image and do anything from producing a nifty newsletter cover to spoofing yourself (free; click on image to see larger version).
6. Also from Flicker, use your cover art or elements from it to create your own motivational poster (I went with more classic art for mine; free).
7. Office Depot will take your cover art and put it on coffee mugs, t-shirts, mouse pads, die-cut puzzles and more (my sister-in-law used them to make some cover-art gifts for me, and I was impressed by the quality of the end product, which is why I'm recommending them).
8. Photo.Stamps.com will create a sheet of customized postage stamps featuring your uploaded image (about $1.00 per stamp; a bit pricey but a nice collectible or gift for your favorite writer).
9. John Pollock's PageResource.com has some interesting web design articles, including one on Resizing Images and How to Promote Your Artwork Online.
10. BellaOnline's Yvonne Russell has an article here with a list of promotional widgets marketing sends out; she suggests authors make -- you guessed it -- bookmarks.