WordNet sense 1 (without payment):

volunteer

WordNet sense 2 (of your own free will or design; not forced or compelled):

voluntary

WordNet sense 3 (engaged in as a pastime):

amateur, recreational

WordNet sense 4 (not professional; not engaged in a profession or engaging in as a profession or for gain):

nonprofessional

WordNet sense 5 (not paying a salary):

unsalaried

WordNet sense 6 (owed as a debt):

outstanding

WordNet sense 7 (costing nothing):

gratuitous