RPGNow is having a huge "GM's Day Sale" running this week (March 2 to March 8) and Fantasy Games Unlimited (FGU) has pdfs of their games from the late 1970s and early 1980s on sale. You can pick up a copy of Space Opera or Bushido for $5.99, a copy of Flashing Blades for $4.50, Villains and Vigilantes for $3.36, and more. If you don't have these games -- especially Bushido and V&V -- now is a great time to pick up what appear to be very nice PDF copies at great prices.
Space Opera is a very complex science fiction RPG. It's near the complexity level of Chivalry & Sorcery, which isn't surprising considering the two games have Ed Simbalist as one of their designers. Space Opera is 180 pages of old school goodness. You may never play it, but you'll borrow stuff from it for Classic Traveller. (FGU no longer has the rights to Chivalry & Sorcery, which is a shame as it would probably be on sale too.)
Flashing Blades is a game of swashbuckling adventure in France. It's GDW's En Garde remade into a full RPG. Short rules, but a lot of fun to play, especially for fans of the Three Musketeers.
Bushido is probably the best Japanese Fantasy RPG ever produced. The rules are fairly simple for an FGU RPG, but they are playable and really capture the feel of Samurai Japan, where a character's skill at poetry can be just as important as his swordmanship. If fantasy is your game and you can only buy one FGU game, this is the game to buy.
Villains and Vigilantes is the second edition of the second Superhero RPG published (Superhero 2044 was the first). It is still a great superhero game today. Characters are created randomly so one can start playing without spending hours min-maxing a character in a point-buy system. FGU has a lot of V&V adventures available as well.
There you have it, my list of four FGU gamesystems any old school gamer should consider adding to his or her collection.