Phyrexian Unlife

You don't lose the game for having 0 or less life.
As long as you have 0 or less life, all damage is dealt to you as though its source had infect. (Damage is dealt to you in the form of poison counters.)

Death's Shadow

Death's Shadow gets -X/-X, where X is your life total.

Worship

If you control a creature, damage that would reduce your life total to less than 1 reduces it to 1 instead.

Plains

Moltensteel Dragon

({R/P} can be paid with either {R} or 2 life.)
Flying
{R/P}: Moltensteel Dragon gets +1/+0 until end of turn.

Norn's Annex

({W/P} can be paid with either {W} or 2 life.)
Creatures can't attack you or planeswalkers you control unless their controller pays {W/P} for each of those creatures.

Nine Lives

Hexproof
If a source would deal damage to you, prevent that damage and put an incarnation counter on Nine Lives.
When there are nine or more incarnation counters on Nine Lives, exile it.
When Nine Lives leaves the battlefield, you lose the game.

Near-Death Experience

At the beginning of your upkeep, if you have exactly 1 life, you win the game.

Swamp

Shadow of Mortality

If your life total is less than your starting life total, this spell costs {X} less to cast, where X is the difference.