I reviewed Art Authority, this great art catalogue for iPad earlier, and I just bought the same application for the Mac.
The bad news, the user interface is a lot worse than the iPad. You browse art in finder windows, sometimes via HTML pages.
The good news, working with the images is a lot easier. Since a good keyword search mechanism is still missing, a very large monitor makes it easier to browse icons of paintings. You can have multiple thumbnail windows open, and leave them open for a long time.
Ten dollars well spent. Twenty dollars well spent if you buy the iPad app as well.