issue #1 of NOD. This is the seed that morphed into the optional Feats in B&T (some of them didn't make the cut, but I'm sure you could use the list to add options to your own B&T game). Not to mention some additional classes for S&W, especially if you just want to stick with the free S&W Core rules.
What, you can't an issue of NOD? First issue is free and full of goodness for any OSR game.
From the blurb:
In this first issue of NOD, you can explore the Wyvern Coast, a sandbox hexcrawl with over 190 encounters. There are six new classes, three new races, a random village generator, a dozen new deities, new monsters, and more! Compatible with most Old School game systems.
Volo's Guide To All Things Magical -
2 hours ago