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.
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.
OSRIC (Old School Reference & Index Compilation) offers a free fantasy role playing game inspired by first edition Dungeons and Dragons. It’s not the first effort I’ve seen trying to recapture the magic of 1st edition, before the life was sucked out of it with the bloated later editions.
The Farland Mass Combat System is designed to help role players resolve large scale conflicts. There’s also a great — and free — program to help you do so.