Like OD&D, The Fantasy Trip has benefitted from an interest in all things simple, flexible and elegant.
While The Fantasy Trip, being the books Melee, Wizard and In The Labyrinth, and the various adventures and advanced versions of those gamebooks, are out-of-print, and are now available only in the used-game market, The Fantasy Trip lives on under another name: Legends of the Ancient World, which is available as a free pdf from the Dark City Games website.
Legends of the Ancient World (LotAW) is a retro-clone of The Fantasy Trip. Its game mechanics are simple, and are almost identical to those employed by the original The Fantasy Trip game.
For example, there are only three stats in LotAW: Strength, Dexterity, and Intelligence. Each of those three stats begin at 8: you get an additional 8 points to spread between those three stats. Your Strength determines how much damage you can take, and how heavy of a weapon you can employ. Dexterity determines how likely you are to strike an opponent. Intelligence determines the potency of your spells, or the number of skills you may possess. Armor reduces the amount of damage you take.
LotAW truly is "rules-light": the rule-set fits onto a scant seven pages. I have printed the free rules, in booklet style, so that each player has their own copy. While this is a true retro-clone of The Fantasy Trip, there are several interesting additions to the game.
(1) LotAW uses a "plot-word" system to track your progress and modify encounters. For example, you may have an encounter. The results of the encounter will depend on whether you have a related "plot-word" for that encounter.
(2) LotAW has added a number of races that I don't recall being in the original game: Tigrans, Ursans, Caprians, Dwarggs, and Snake-men.
(3) LotAW has added a system of Karma and Wish points that allow players to modify their rolls or change the results of encounters. In addition, there are Curses that appear during play, that affect character rolls or ability scores.
This game is not for everyone. I personally find it a little too simple for a full-blown fantasy campaign, and am not a big fan of using Strength both as an ability measure, and also as your "wound" pool (in TFT and LofAW, wounds are marked off against your Strength). In addition, spell-casting is based on a points-system, with stronger spells requiring more fatigue points, and with spell fatigue also marked off against Strength.
On the other hand, if you are looking for a simple (and free) role-playing and/or fantasy game system, this may fit the bill. I play this game with my kids, and it is a good introduction to role-playing.