If you're looking for similar to Digimon Movie 1: Digimon Adventure, you might like these titles. All recommendations are made by Anime-Planet users like you!
In the future, all facets of society are tied into OZ, a virtual world inhabited by millions of users. Kenji, one of OZ’s moderators, was set to begin another typical summer when the lovely Natsuki asked him to accompany her to her hometown as a job. However, little did Kenji know that the 'job' entailed pretending to be Natsuki’s fiancé in front of her eccentric family! Now on display and feeling like a fish out of water, Kenji tries his best to fit in with Natsuki and her relatives, until one day he receives a mysterious math problem through a text message. As an avid math fanatic Kenji can’t help but try to solve it, unaware that his actions may jeopardize not only OZ, but also the entire world...
Plots are very similar. Wars in the digital world. Both are funny and have a great art style to them as well.
(Have in mind that Digimon movie one is combination of movies 1 and 2, or at least it was back then when I watched it) First of all I think that Summer Wars is massively influenced if not copied Digimon on a whole new level. If you watch both movies in a row you will notice that SW copied a lot of scenes and even dialogue. I don´t think that its dry copy but its really hard to ignore that much similarities. In both cases plot is practicly the same... shocking? Difference is in charachters and world but everything else from plot development to major twists is too damn similar (if not same). I hate to be the one impying that one thing is copy of other, but in this case it is hardy ignorable. Both are marvelous anime, however SW is newer and more appealing to watch for a new generations (mostly beause of animations and possibly better action, which I doubt). But to me Digimon the move (Movies 1 and 2) is far more superior anime. If you enjoyed one of these you should definately grab the other one.
Both of these series are very similar in a variety of ways. While the Digimon movie does come first, and Summer Wars is almost plagarising Digimon's plot, both movies are equally amazingly well written and animated.