www.fictionwise.com
www.kobobooks.com
Fictionwise is usually more expensive than amazon and Sony *but* they have a rebate system and frequent coupons that if used properly will net you very cheap ebooks. I'm averaging <$3 per ebook at fictionwise right now and that includes many books I spent $17 when they were at 100% rebate, to get the Micropay balance to pay for other books.
The key to fictionwise is only buying books when they are on special sale -- when you can get a rebate even paying with rebate dollars. Currently the Agatha Christie books are on 75% rebate and there is also a 25% off coupon this month, so that the final price of the books is <$1! The system is a bit complicated, but once you get used to it, you get really great deals.
The one caveat is you'll really need to learn to strip drm from your ebooks and convert them, as most aren't available as epub there. If you google around for darkreverser you should find some help.
Kobo is more expensive, but they have some deals, such as most of the Mickey Spillane books very cheaply.
-Marcy