If you're looking for anime similar to Chaos;HEAd, you might like these titles. All recommendations are made by Anime-Planet users like you!
Middle school student Ganta Igarashi witnessed the slaughter of his entire class by the mysterious 'Red Man', and as the only survivor, he was labeled a mass murderer and sentenced to death. Now an inmate at the privately-owned prison and ‘amusement park' known as Deadman Wonderland, Ganta must try to survive in a place where inmates are the main attraction in brutal gladiator-style games. At the same time, he must attempt to uncover the truth behind the ‘Red Man', his mysterious childhood friend Shiro, and why he was the only survivor that bloody day.
Medusa is a mysterious illness which causes the body to petrify shortly after infection, found throughout the world and steadily increasing the death toll. In order to combat the disease, 160 lottery winners were chosen to be frozen until a cure could be found, one of whom is Kasumi - a twin whose sibling was not one of the chosen. Along with a group of others Kasumi awakens from her cryogenic chamber, but not to the same world she left it: prehistoric monsters roam the now-abandoned castle and eat the survivors, while endless giant thorns rise all around them. Against all odds, Kasumi and the others must now try to survive the horror of their situation and discover the truth behind what's happening, all the while waiting for the Medusa to finally claim their lives...
In a massive online world, people act only as well as their conscience demands. When one player's punishment is to be trapped in this world, a disparate group of people seek the answers why. Their motives are varied; their methods even more so. What stands between when we tear down the walls of reality? Where does our soul end and we begin?
This is sort of a weird recommendation - Chaos;HEAd deals a lot with delusions but it happens in the "real" world, while .hack//SIGN happens in a MMORPG (ironically called "The World"), so their settings do set these shows apart.
With that said, Chaos;HEAd has a ton of ideas and terminology that relates back to the digital world, with characters wielding "Di-Swords," and other references to computers and games. While Chaos;HEAd has a plot that some say might is overloaded with ideas, if you liked some of the ideas involving mysteries and delusions presented in Chaos;HEAd, I'd suggest checking out .hack//SIGN.