Here is a list of interesting deals we found on DriveThruRPG for their D&D May sale (non-affiliated links).
Core rules books:
D&D Rules Cyclopedia (Basic), $5.99 instead of $9.99;
Adventurer Conqueror King System, $6 instead of $10;
Beyond the Wall and Other Adventures, $4.79 instead of $7.99.
OSR:
Wormskin Issue 1, $4.50 instead of $7.50;
Slumbering Ursine Dunes, $5.40 instead of $9.
Other supplements:
DM Yourself - solo roleplay for 5e, D&D and OSR adventures, $5.56 instead of $6.95;
The Halls of Arden Vul Complete, $65.40 instead of $109.
Don't hesitate to share your favorite products or best offers found on DriveThruRPG in the comments below :)