Microlite 78 First Edition Lite - This time, only 500 pages. $4.95
RPGPundit Presents #37: Advanced OSR Critical Tables - I don't think critical hits are particularly OSR, though admittedly a common house rule, plus the whole Arms Law thing from ICE. But as EGG pointed out, they end up hurting the players more (since they will get hit more) in the long run. $1.99 and 16 pages.
Friday Enhanced Map 6-22-18 - A keep or stronghold. $1