Right now every Wizards of the Coast D&D product at DriveThruRPG is now on sale.
And there are some great deals.
D&D Rules Cyclopedia (Basic) for $5.00
Seriously, if you don't have this grab it now. Just do it.
Rules Compendium (3.5) for $7.00
For D&D 3.5. I don't even have this (was burned out on 3.5 when it was published) but I am grabbing it now.
D&D Rules Compendium (4e) $5.00
I have a post coming up that the 4e might find a new life online thanks to cheap PDFs. This is certainly a good place to start.
I would also like to point out the World of Greyhawk Fantasy Game Setting (1e). This thing is really high priced on eBay, but here it is only $8.50.
Grab these and more now.