Digimon Adventure: Our War Game
A year after the events of the first season, a virus Digimon called Diaboromon appears and starts attacking the Internet. The children and their Digimon unite once more in order to prevent him from destroying every communications network on the planet.
A personal favorite back when I was a kid, however I later found out the english version was heavily edited from the original japanese and the story didn't even match up the same. Supposedly there were elements taken out that had an impact and tied everything together correctly.
Really neat -- I love how the internet is represented as a physical space in this one. You can really see how it paved the way for Mamoru Hosoda's later film "Summer Wars," which expounds on a lot of ideas laid down here.