SJ Games is preparing to release their first new GURPS book in a year. It is Low-Tech a summary of technology from the Stone Age (TL 0) to the Renaissance (TL 4). The PDF has been released on e23. While somewhat rules heavy it is an excellent reference and can be used as a source of equipment for just about any Fantasy RPG. For D&D and retro-clone, the pricing would convert at roughly $4 for 1 D&D silver piece. This book will followed by three smaller PDF releases dealing with advancements in government, warfare, and economics.
To me the Old School Renaissance is not about playing a particular set of rules in a particular way, the dungeon crawl. It is about going back to the roots of our hobby and seeing what we could do differently. What avenues were not explored because of the commercial and personal interests of the game designers of the time.
What are RPGs?
A game where the players play individual characters interacting with a setting with their actions adjudicated by a human referee.
Rules are an aide to help the referee adjudicate actions and to help the players interact with the setting.
Dice are used to inject uncertainty which make a tabletop RPG campaign more interesting than "Let's Pretend".
The only thing a player needs to do to roleplay a character is to act if he or she was really there in the setting in that situation.