Hands down, the best single non-TSR supplement for Original Dungeons & dragons is the second edition of the Judges Guild Ready Ref Sheets booklet. In not only has combat tables, but a double-sided sheet with every standard OD&D monster summarized in a couple of lines and all the rules suggestions from the first six guildmember installments. Things like the social level and justice system from the City-State, ideas for advertising for hirelings, poison, enchantment, trade, caves & lairs, searching, prospecting, and much more. I rediscovered my copy last night. It’s great stuff.
It’s available in PDF form from DriveThruRPG.com for $2.99 (on sale for $1.99 as I write this). Assuming the reproduction is good, that’s a great price as my original was $2.99 back in 1978!