There are two waterborne urethane finishes commonly used by flooring refinishers today: Bona and StreetShoe. Both are said to have revolutionized the urethane industry by offering extremely durable waterborne finishes. Both products are used in commercial applications where high-traffic wear is common. I have used neither but have spoken with many professionals who swear by these products. Both are finishes that create a plastic-like coating on the wood; over time, the coating does wear down and flake off at the joints as the wood floor shrinks and expands from season to season. Bona and StreetShoe claim to meet the 250 g/L (grams per liter) VOC regulation imposed by most states.
However, the product I'm most familiar with is Osmo Polyx-Oil. (Full disclosure: I've chosen to sell the Osmo product at Wood Anchor, based on the advantages I describe below.) Osmo's Polyx-Oil is a natural finish made from vegetable oils and plant waxes. It is an oil-based product that is also VOC compliant. One of the reasons we use it on all our floors is that it is as low-maintenance as any other finish, but it is also repairable. Any scratches can be spot-repaired without having to fix your whole floor. This means that with proper maintenance you will never need to refinish your floor again! Unlike urethane and polyurethane finishes, Osmo is not a plastic coating sitting on the wood. Osmo Polyx-Oil becomes part of the fibers of the wood and fortifies the surface of the wood. Because it doesn't sit on the surface, it is guaranteed to never flake or peel.
One of the downsides of Osmo Polyx-Oil is that it only comes in a matte finish. Also, because fewer retail outlets sell Osmo than Bona and StreetShoe, many people end up ordering Osmo online. As the popularity of Osmo grows in North America, so too will the number of stores carrying it. Often it's best to call eco-friendly stores in your area and ask if they have it in stock. Those local retailers may also be able to provide referrals to installers who have experience with Osmo.
Installed cost per square foot is very similar for Bona, StreetShoe, and Osmo, varying only about 10 percent depending on the application technique. One of the key differences between application of Osmo and urethanes is that Osmo is applied as thinly as possible; a thicker coat will not protect your floor more. While Osmo is typically the most expensive per volume of product, the installed cost will be comparable to Bona or StreetShoe.