 If you liked Serial Experiments Lain |  ...then you might like Summer Wars |
Although quite different in style, the concept of a cyber world is paramount in both of these animes.
In both shows concept of cyber world plays an important role in the storyline. While Lain has Wired, Summer Wars has Oz. Both are involved with a mystery.
Both of them deal with the Internet and avatar character representation of the characters who are on there, though Summer Wars has done it better with less complexity and maturity than Lain.
report Recommended by Angry_Anime_Nerd
As far as anime portrayals of the internet go, these are two sides of the same coin. Summer Wars takes a modern, optimistic approach to portray a connected world fostered by personal relationships. Lain takes an introspective, alienating approach to portray a blurred reality where despite being connected, meaningful connections can seem incredibly distant.
Although SEL is much, much darker than Summer Wars, and the moral of the story is very different, plot-wise, they are surprisingly similar. They are both sci-fi anime (although Summer Wars is also slice-of-life and romance), and feature a world where everything revolves around something very similar to what we call the Internet. In Summer Wars, that thing is called OZ, and in SEL, it is "the Wired". When I saw how in the world of Summer Wars, you could select avatars and travel inside the system, and especially when they emphasized how much the real world was tied to OZ, I immediately thought of