I've gotten some really good ones. Roughly in order of how well I liked them:
Tournaments, Fairs and Taverns is excellent. Reading it made me want to run it. My only real issue with anything in it was a sentence stating that teamwork wasn't very useful in poker, but they give a rule to make it useful in the section before that sentence, so I wasn't too offended.
Magical Medieval Society is a good workbook, although not quite as good - they used the D&D demographics and economic system, which hurt my estimation of the book. Still, they give a fairly clear path to reverse engineer and use real numbers. However... this is NOT a plug-in resource, it's a builder's resource for world-building. Still, at under $10, it was a no-brainer for me

.
3 Arrows for the King is a good archer's resource. They have some twinky feats, and some not-as-good feats, but the archer class is pretty well balanced, and the rest of the resources and commentary on archery is well taken.
Elements of Magic hasn't really captured my interest yet (I got it because of the GM's Day Sale: $2.50), but it's decent quality - I just tend to build my own magic systems anyway.
Group Feats is a few nice new feats to stick into a campaign. It costs $0.60. I can dig.