Steins;Gate[1] contains references to John Titor[2]. I actually thought they had made him up until I looked on Wikipedia and discovered he was a real ("real") conspiracy theory.
I can't speak to the anime, but I enjoyed and recommend the visual novel version.
I got into it because I wanted a break from more traditional games (such as RPGs) which may claim to have a story but often don't go deep into character development and so on. If that's what you want, Steins;Gate definitely delivers.
I'd make two cautionary points if you're going to try it:
1. It's a visual novel, so it's not going to be interactive like a more traditional game. You get some choices, which among other things determine what ending you get, but really don't influence much otherwise.
2. There's a lot of banter. You'll know in the first 10 minutes of the game if this bothers you or not. I personally didn't have a problem with it, but your mileage may vary.
I really enjoyed it and I would highly recommend it. It had the deep philosophical aspects to it but has a decent amount of humor to go along with it as well. I have been recommended the visual novel which i haven't read though. There is also a movie to go along with it.
https://en.wikipedia.org/wiki/Steins;Gate#Anime
The visual novel is really fantastic, though fairly slow-moving if you're not used to the medium. (Visual novels in general tend to be very detailed and very slow-paced. Some, like Higurashi, use this to great effect, but it can also annoy people not used to them.)
If you're not sure about reading it, the anime is also really good. It's obviously much less detailed than the visual novel (compressing ~40 hours of visual novel into TV format is hard) and omits some sizable plot points, but it's still great. The first half is fairly slow set up before it hits like a truck.
For visual novels? Probably Root Double, Ever17, and Umineko. All have really great character development like Steins;Gate and are pretty thoughtful. Root Double has well-done sci-fi elements as well.
