DriveThruRPG has a sale for up to 25% off (30% off would be their best theme sales). Browsing by default sort (popular):

Savage Worlds Adventure Edition ($10 on sale $7.50) - I have this but not on an online store (or the record was lost) so can't update it. Tempting to buy it so I can keep an updated copy (not vital though since errata files are free and errata is fairly minor).

Old-School Essentials Classic Fantasy: Rules Tome ($20 on sale $15) - This is a D&D B/X retro-clone covering levels 1 to 14. Took me a while to figure out the Advanced rules retro-clone AD&D 1E so not necessary if you only want B/X goodness. OSE is I think the most polished D&D retro-clone but probably also the most expensive.

D&D Rules Cyclopedia (Basic) ($10 on sale $7.50) - Since I have OSE on the list I will also mention the best authentic D&D ruleset which covers BECMI except the Immortals. Mind you few campaigns get beyond Expert Set but I'd rather have rules going to level 36 and not use them than not having the rules and needing them.

I got tired after the first 750 entries and didn't see anything else interesting.