Temple Gates Games
Dominion is an official online implementation of Dominion, produced by Temple Gates Games. It is available on multiple platforms, including Steam, and various app stores.
Features
All released cards are implemented on the TGG client. Games can be played against either one of three difficulties of AI, or another human; currently only 2-player games are implemented, but there are plans for higher player count games. The AI has been touted as a headlining feature of the client, claiming to be taught via neural networks, and receiving some press upon its announcement.
Payment is one-time, per expansion, rather than the subscription model that Dominion Online uses. The developers are honoring purchases of expansions made through previous platforms (Goko and Making Fun), though those clients only ever implemented up to Adventures.
Cross platform play SteamPC + Android + iOS. Purchases can be shared across platforms by entering an email in Profile->Online Account->Add Email.
Currently the client is only available in English.