WordNet (r) 3.0 (2006):

labor of love
    n 1: productive work performed voluntarily without material
         reward or compensation [syn: labor of love, labour of
         love]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "labor of love":
   act of grace, act of kindness, benefaction, benefit, benevolence,
   benignity, blessing, complimentariness, costlessness, courtesy,
   expenselessness, favor, free ride, freebie, freeness, gift,
   good deed, good offices, good turn, grace, gratuitousness,
   gratuity, kind deed, kind offices, kindly act, kindness, mercy,
   mitzvah, no charge, obligation, office, service, turn