If you're looking for anime similar to Alien Nine, you might like these titles. All recommendations are made by Anime-Planet users like you!
"I have only abandoned my body, I still live here" - are the words emailed to friends of Chisa, several days after her death by suicide. As Lain delves deeper into the world of the "Wired" (also known as the internet), the line between it and reality becomes more and more unclear. Close the world, open the nExt.
Both Lain and Alien Nine are scary, quirky, and off-beat. Both series are also brief but well-executed story-telling gets the point across. (Alien Nine doesn't have a definitive ending.) Both stories center around a timid girl put in bizarre situations, and nobody else seems to notice.
Also, if you really appreciate clever soundtracks then you should watch both of these series. They're total opposites but they're very effective. Lain sounds like electric buzzing, city sounds, and techno; Alien Nine sounds like a magical girl show with happy J-Pop despite the horror on-screen.
A hyper girl named Azuki Edomae and her friends Mutsumi Mutsuki and Serika Shihōdō protect the peaceful order of a town by transforming into Kigurumikku-Swan, Kigurumikku-Falcon, and Kigurumikku-Swallow, respectively, via cosplay outfits. "Kigurumi" is a type of cosplay that essentially turns a wearer into a giant plush toy or doll.
Main reason for this recommendation is the somewhat highly paced action scenes and the main theme of "wearable" aliens. The main characters in both shows are also on the very young side. Both shows have comedic elements, but it's the main focus in Kigurumikki (and barraged at you ni high tempo), whereas it's just one of many genres in Alien Nine.
Provided you like the premise of Kigurumikku, but want something more serious, check out Alien Nine. If you enjoyed Alien Nine and if you are also a fan of random comedy, Kigurumikku might be an interesting show to check out as a short snack.