If you're looking for anime similar to Digimon Season 1: Digital Monsters, you might like these titles. All recommendations are made by Anime-Planet users like you!
Kimimaro Yoga could use a break. At nineteen years old, he's not only a student at Heisei College of Economics, he's also a part time employee and flat out broke. So when an eerie man offers the boy a special ATM card and an exorbitant amount of cash, Kimimaro gives in to temptation – but there's a catch. In exchange for his good fortune, Kimimaro's very future is put at stake, held as collateral by the Bank of Midas and tied to the amount of yen in his bank account. In addition, he must participate in a special battle every week in the mysterious 'Financial District' – a battle where losing against one's opponent can mean bankruptcy, a fate that carries an unthinkable cost in the normal world...
If you grew up watching mon fighting shows and want something similar but for older audiences you should watch C. In C the characters have cash cards with a creature inside called an asset which they use to fight each other for money.
The year is 1969, and strange things are happening on Earth: a strange formation known as Brigadoon has appeared in the sky, and mysterious alien machines have begun to attack a seemingly ordinary orphan girl named Marin! Luckily for her, Marin finds an artifact which summons the alien Melan Blue to protect her – but there is more going on between Earth and Brigadoon than meets the eye. However there is more going on behind the scenes between Earth and Brigadoon. Along with a cast of quirky characters, Marin must get to the bottom of what’s going on, lest both Earth AND Brigadoon be destroyed forever!
The later parts of Digimon Adventure have much in common with Brigadoon: Marin to Melan. In both shows, our protagonists are children who look to the sky and, to their surprise, can see a strange "world" in the sky, and that world happens to be inhabited with colorful and strange creatures. Digimon Adventure, of course, is a much tamer show meant for a younger audience, while Brigadoon often falls into grim territory. If you liked the setting and world concepts in Digimon but want something darker (and often funnier), then give Brigadoon: Marin to Melan a shot. I promise you won't be disappointed!