The Collaborative International Dictionary of English v.0.48:

Heartfelt \Heart"felt`\ (-f[e^]lt`), a.
   Hearty; sincere.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heartfelt
    adj 1: earnest; "one's dearest wish"; "devout wishes for their
           success"; "heartfelt condolences" [syn: dear, devout,
           earnest, heartfelt]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "heartfelt":
   absorbing, acute, ardent, bona fide, committed, dedicated, deep,
   deep-felt, deepgoing, devoted, earnest, fervent, fervid, genuine,
   hearty, homefelt, honest, indelible, keen, passionate, penetrating,
   pervading, piercing, poignant, profound, serious, sharp, sincere,
   true, unfeigned, wholehearted