From time to time I try to talk about other Magic programs that you probably haven't heard of and Incantus is one of the best. Incantus is a computer program that lets you play against other people over the Internet and it enforces all of the rules like Magic Online. Incantus has a great user interface that is comparable to the new XBox game. Unlike Magic Online, Incantus does not have a central server so you have to find people to play against although it does have its own IRC Efnet channel #incantus.
Incantus is written in Python and works on Windows, Macintosh, and Linux. And if you really want to get involved you can program new cards or test cards to make sure that they are working correctly, just ask in the forums and they will help you. Incantus also features ALL of the phases and phase stops, unlike my own program MTG Forge. Incantus has more than 5,000 cards. Let me try to put this in perspective, 5,000 cards is INSANE since there are only 10,000 Magic cards ever printed.
Incantus doesn't have a deck editor but you can use Nate's Deck Editor to create decks. Nate's Deck Editor is intended to be used to create any type of deck (constructed, vintage, casual) and includes a listing of all Magic cards. It has a great search feature and can also be used to create decks for Magic Online, Apprentice, or Magic Workstation. Nate's Deck Editor is written in Java and will run on any platform.
And as a side note, Incantus doesn't let you play against the computer. If you want to play against the computer, you have to use MTG Forge.
The password to download Incantus is "rare".
Nate's Deck Builder
Here are some notes for using the deck builder to create decks for Incantus.
When using the deckbuilder you select "Custom" and any cards that are not implemented in Incantus will come up highlighted in red (i.e. they are "banned"). There are 5000+ cards supported by Incantus now and this just makes it easier to know which ones are implemented while using the deckbuilder. Also I recommend saving your decks in Apprentice format because that file format is supported by Incantus.