You can’t get the Fantasy Trip any more, but you can get Dark City Games’ Legends of the Ancient World, a free tactical RPG that uses the same mechanics.
Labyrinth Lord is a free “throwback” style RPG.
I’m old school, so I’m going to give this a serious look.
RPG Now has a free horror RPG module called Dead of Night: Night of the Santa Claws. It’s designed for the Dead of Night RPG, but no doubt could be modded for any other system.
Mobsters is a set of roleplaying rules for 1920s gangsters. It’s a well-done and complete game.
Castles And Crusades is old school Dungeons and Dragons, with some clever systems that both simplify things and make the game compatible with all of yoru old (monster manuals and such. I don’t normally link to commercial products here, but I’m particularly taken with this one.
There’s also a growing community offering free supplementary materials, like the Netbook of Classes. The Netbook offers fifteen new classes, including executioner, harlequin and lore warden.