I can't think of a game this good that so few people have heard of or played! Especially considering that it has a free version available with a ton of content... What's stopping people?

Because it's not fun?

This game is for the perfectionist/highscore crowd and that is not what the majority of gamers are today.
I played it quite alot but it's so hard to just play it for a little bit of fun if you don't know the tracks inside out or didn't play in a while. The concept is really well executed, though.

TMU really is a special game.
 I never played it offline much, but the multiplayer is amazing. I used to host a server that was the only one listed for Norway at the time, and we had some regulars on it. The pure fun of sharing tracks, sharing tips on how to do corners best and just racing against others is something I have not seen in any other racing game.
 
I am usually never a high score guy or a  perfectionist , I hardly ever finish single player games, but in TMU I could drive the same damn track probably 50+ times to try shave off some tenths of a second. I wasn't amazing by any means, some of the people in the trackmania league would come and visit the server sometimes and beat my hard earned times on their first and second try, but that just made it so much sweeter those few times I had a better time than them for a few days.
 
I mostly liked/played the Original track types (desert, rally, snow), the free stadium type was cool too, but the sunrise environments I never liked much. I've not played TMU for a while now, but along with TF2, EverQuest and WoW it is the only game I have played for an extended period of time.

I love Trackmania.

I've played some of the free version recently and I enjoyed it.

I wouldn't say I'm a big highscore/perfectionist dude, and I detest sim racers like Forza or Gran Turismo, but I love Trackmania... still not entirely sure why, it's just good.

I played a ton of TMNations and enjoyed the hell out of it. It got really fucking hard though. 

I'd say Freedom Fighters or Stalker: Shadow of Chernobyl. The latter definitely had some major bug issues, but it's atmosphere was ominous, at times terrifying and very engrossing. It's (at the time) great lighting effects helped things; it wasn't what you could see, it was what you couldn't see. I remember spending about 5 minutes (I was 15, although it would still probably take me that long now) gathering my wits to open a door that was being bashed from the other side.

I've played over 100 hours of Trackmania Nations Forever, and even I can say that I don't really think that's a very good game. I just played it when I was really bored. Like, it's cool and all, but definitely not the most under-appreciated game ever. 
 
EDIT: Actually, saying it isn't good is really harsh. It's a really damn fun game, but as I said, not quite worthy of the title you give it.

I played it a bit when it was released, because someone told me it's like Stunts (4D Sports Driving), which is the first racing game I ever played and loved. Unfortunately, even though Trackmania is a great game, I somehow always lose interest after a short session. But it is undoubtably a game you can have a lot of fun with even if you don't play it that often.

I've been playing Nations for ages, I'm still considering if I should just get United or wait for TM2...