Nicolas Caillard's Life Counter for Magic: The Gathering is utterly useless for the majority of Windows Phone users, but if you lament the days of Baron Sengir, then it's going to take pride of place in your smartphone

Allowing you to track the life of up to six players in the popular trading card game, it has a few features that will make it more useful than the lo-fi solution I use (a d20 being turned to the right number). The main one is the use of poison counters, which can be added and removed by toggling between life and poison. The other one I like is the "coin flip" option for those heads or tails moments the game likes to offer!

It's a free download from the Marketplace.