Dominion Online at Isotropic.org is a rather cool way to play Dominion online. It’s very elegant, all monospaced text, and works like a charm. It has all cards, every card from every set and promos too. The user interface is mostly intuitive, and certainly better than what Brettspielwelt has to offer.
I used it to get some feeling of Prosperity. Prosperity is cool and definitely ups the ante. It wasn’t rare to see 15-card, multibuy hands in action. If you like that sort of thing, Prosperity is your expansion. I like it.