Seedling pot made from an empty toilet paper roll. After germination, it can be buried directly in the garden bed as is; the roots will grow through the softened cardboard, and soil organisms will then consume it. Make sure to only use toilet paper rolls made from unchlorinated and unprinted cardboard.
edit: please check if recycling has a problem with chemicals or heavy metals in your country
You can make pretty good ones using layered newspaper. Or any paper that doesn't have something nasty in it. Using a cylindrical glass or similarly shaped aid. I've used a big vitamin bottle with the top sliced off. There are a bunch of tutorials.