Rebirth

Acquisition
Skill Quests: Skill Trainers: Signet of Capture:
 * Dropping Eaves (Ettin's Back)
 * Prophecies
 * Tengsao (The Amnoon Oasis)
 * Dakk (Ember Light Camp)
 * Factions
 * Michiko (Kaineng Center)
 * Tree of Life (Bloodstone Fen)

Counter Claim
After the release of Factions it has been suggested that the Ritualist has a number of spells superior to Rebirth. See Flesh of My Flesh, Lively Was Naomei, and Restoration to consider the argument.

Additionally, for non-casting classes like a Warrior/Monk build, the factions spell Resurrection Chant offers an excellent res skill for 8 seconds of cast time. While this is not ideal in PvP situations, in PvE situations where a caster goes down, a tank can res that ally to full health and a bit of energy, with no skill lag.