A million years ago, I used to play MTG online against friends to test decks for tournaments using a program called Apprentice.
12 years on, it's still around.
- Grab "Apprentice" from that list. It's 1 meg.
- Grab "Rise of Eldrazi Oracle Patch" from this thread.
Install Apprentice. Unzip the card database into the same directory. It'll ask to replace files.. replace all. Load it up and play.
It's not very pretty, but it's efficient.
Build your decks, tune and play. =D
You can play "single player" to test draws and stuff. But you need to organize games on MSN, forums or whatever. You generally give your opponent your IP address and connect.