The first publisher of OSRIC adventures and supplements is having a 60% sale to celebrate their 20th anniversary. Plenty of good stuff to wade through. Here are some of my favourites.
A Magical Society: Beast Builder. Atlhough for d20 system, this isn't some “roll X dice randomly, get a monster.” Like other books in the series, it offers a detailed take on how to create beliavable, exciting, and dangerous monsters that fit into your fantasy world.
The creators of Castles & Crusaders are discounting their whole catalogue until end of the year. Although I don't run their system, I use some of the adventures as well as system neutral aids.
Lost City of Gaxmoor (digital and isometric maps). A city that was gone for a thousand years reappears. Of course it's full of shit that wants to kill nosy adventurers. Plane of chaos breach the reality from time to time.
Chaos Touched. Additional random encounters for chaos breaches. Intended to be used with Gaxmoor, but can be adapted for any area.
Undercaverns of Gaxmoor (maps). This one progresses timeline quite a bit, returning Gaxmoor into the hands of good. I just used it to steal what is underneath.
Engineering Castles. Another brief, but useful supplement. It does not generate the internal floor plans, but rather the type of castle, inhabitants, and environs.
I've just seen that Frog God Games is running another sale with discounts up to 70%. As before, most of my suggestion below are for the Swords & Wizardry system, a retroclone of Original D&D.
In no specific order, I can recommend the following:
Monstrosities (Swords and Wizardry). Nearly 500 monsters. Each monster comes with an example encounter/nano-adventure. Includes tables with monsters by challenge level, guidance on creating new monsters, tables of monsters by terrain, and tables of random encounters (3d6, so bell curve).
The Northlands Saga Complete (Swords and Wizardry). A compilation of ten adventures set in stereotypical cold north. Probably enough for several years of gaming. My favourite activity is stealing from this book and including parts of it in my own game. Tenfootpole has reviewed first four adventures back in the day (NS1, NS2, NS3, NS4). $18 is a steal for this.
The Lost City of Barakus (Swords and Wizardry) (local and regional maps). Perhaps my favourite Necromancer Games mega-dungeon—because it is so much more! You get a starting city (with seven adventures), a wilderness area (with 26 keyed encounters and mini adventures), and a mega-dungeon with interesting factions and cool big-bad. Suitable for low-level parties.
Bard's Gate (Swords and Wizardry) (player's guide). A massive city packed with urban encounters and adventures (8 included, from levels 1 to 10+). Very dense book. Some say this is FGG's finest product. The truth is that this is another Necromancer Games revival. And that's why it's great. :)
The Book of Taverns (volumes one, two, and three). Had enough of generic taverns and inns, but short on prep time? Steal one from here. Again, these are revivals of old Necromancer Games books. They are good.