Well, big in the expansive sense, not really deep discounts, seemingly only 20%. They call it a "D&D sale"
OSR participants include: Expeditious Retreat Press (Advanced Adventures) , Grey Fey (a bird adventure), A. Hagen (LL sourcebooks), OSR Dan (sourcebooks for LL), Dark Naga (adventures), Three Sages (a couple of nice adventures), Splintered Realms, Hawk Wolf (2e sourcebooks) , Gallant Knight (the aborted Adventures in the East Mark set and Mazes & Perils adventures), Okumart (Gramma World and some adventures) and Infinibadger Press
Possibly more, but those are the companies whose work I have is on my wishlist.
One is some of James Mishler's stuff which is generally quite reasonable to begin with.
Another is Sine Nomine whose stuff I already have. Usherwood makes OSRIC sourcebooks/adventures. And Jeff Dee's Unigames has two very short adventures which probably leave "more OSR Than thoug" types aghast at the amount of treasure, but accurately represents actual old school design.
Lastly, Spes Magna Games is having a 25% off sale all month in honor of the guy's wedding anniversary. He has a number of adventures and some misc stuff.