Sourcebooks. Some are bad, some are good, most are indifferent. Here are my favourites.
1. Pacific Rim Sourcebook for Cyberpunk 2020. There are lots of flaws in this book, not least that the section on Japan is far too long and those on the really interesting countries (Korea, the Phillippines, Indonesia, Myanmar) are far too short. But it does a brilliant job of setting out a detailed, cohesive, believable futuristic Pacific Rim - not at all far-fetched, and with just a touch of imaginative flair. The post-apocalyptic Australia in particular is a great Mad Max rip-off and a perfect place for a down and dirty Cyberpunk 2020 campaign. The book also contains an expansion for the detailed martial arts rules for which the game is rightly famed, together with glossaries of major East Asian languages (although I'm not sure how accurate they are if the Japanese one is anything to go by) and lots of new guns.
2. The Planewalker's Handbook for 2e AD&D. This is definitely my favourite TSR-produced sourcebook, mainly because I love its scrapbook, ad hoc approach, where on one page you'll find rules for Genasi player characters, on the next baatezu green steel, then a kit for wizards who fly around in hot air balloons after that... It's like a treasure trove. Not only that, it contains the rules for Belief Points, which were one of the major innovations in late-era TSR D&D and just about perfect for the Planescape setting.
3. Changeling: The Dreaming, The Player's Guide. Changeling: The Dreaming was always the most interesting of the World of Darkness settings (in fact the only one I ever really liked), and the player's guide was like the icing on the cake - more detail about the major races, plus a whole shedload of material on Native American fae. Best of all was the final section on the Autumn People; the idea of individuals who can kill fae through their sheer banality was way more scary in its own way than anything in Vampire or Werewolf.
4. The Orcs of Thar, for BECMI D&D. Evil humanoid races as player characters for classic D&D. Need one say more? Well, throw in rules for humanoid shamans, a ridiculous board game that nobody ever played, background info on all the races, and sheafs of maps. What more could a person want?
5. The Northwestern Middle Earth Map Set, for MERP. One of the cornerstones of being a Tolkien fan is the love of the geography, and one of the main pleasures of that occupation is poring over maps and imagining what might be found in the places illustrated. This book fulfils that need to connect with Middle Earth cartography in style.