The most sustainable option in flooring is clearly making use of what you have. But that's not always possible or desirable.
Once we removed the old, funky-textured linoleum, peeling asbestos tiles, and avocado green and royal blue 1970s carpeting, we were lucky to find good oak wood flooring throughout most of our new home, so we'll only have to sand, then finish them with a low- or no-voc wood floor sealer.
Our kitchen and bathrooms are exceptions to the warm oak flooring, so we considered other eco-friendly options and for the kitchen we settled on marmoleum by Forbo Flooring, a "green" version of linoleum. The tiles and wide strips are a composite of recycled materials, including wood, flour, and "tall oil," and renewable products, including jute, linseed oil, and pine rosin.
I like marmoleum in the kitchen for many reasons, not least of which is that it's easy to clean and is kind on the feet and body when you're standing and working there for long stretches. We also loved the wide color palette it's available in and that you can cut, mix and match tints to achieve some very fun, creative results.
There are many other eco-friendly flooring options, including bamboo, salvaged wood or tiles, concrete, recycled glass and ceramic tiles, recycled rubber, cork, renewable, natural fiber rugs and carpets, and others. Green- and aesthetic-wise, considerations are many when you shop for eco-friendly flooring. Not all the choices are equal when it comes to their environmental impact, so do your research and make the best choices you're able to make.