Forge - Slightly Magic. Really practical software for Magic beginners like me. You can assemble decks, use ready-made decks, get randomized decks - and then play them against an AI. The AI isn't the strongest, but hey, I'm far from that too. It's all in Java and actually runs on Windows, OSX, and Linux alike. Nice to give your own deck a test run, especially if you don't have Magic players nearby (or they don't feel like testing your crazy creation). Oh, and it's open source too.