WordNet (r) 3.0 (2006):

guts
    n 1: fortitude and determination; "he didn't have the guts to
         try it" [syn: backbone, grit, guts, moxie, sand,
         gumption]

Moby Thesaurus II by Grady Ward, 1.0:

157 Moby Thesaurus words for "guts":
   abdomen, anus, appendix, backbone, being, bite, bitingness,
   blind gut, bones, bosom, bottom, bowels, brain, breast, briskness,
   cecum, census, chutzpah, colon, composition, constituents, content,
   contents, courage, cuttingness, decisiveness, divisions, drive,
   duodenum, effectiveness, elements, endocardium, endurance, energy,
   entrails, esprit, force, forcefulness, foregut, fortitude,
   gameness, giblets, ginger, gizzard, grit, gutsiness, guttiness,
   hardiness, heart, heart of hearts, heart of oak, heartiness,
   heartstrings, hindgut, hotness, impressiveness, incisiveness,
   index, ingredients, inmost heart, inmost soul, innards,
   inner mechanism, innermost being, insides, internals,
   intestinal fortitude, intestine, inventory, inwards, items,
   jejunum, kick, kidney, kishkes, large intestine, list, liveliness,
   liver, liver and lights, lung, lustihood, lustiness, mettle,
   mettlesomeness, midgut, might, mightiness, mordancy, moxie, nerve,
   nervosity, nervousness, nip, nippiness, obstinacy, part, parts,
   pepperiness, perineum, pith, pluck, pluckiness, poignancy, potency,
   power, powerfulness, pump, punch, pylorus, raciness, rectum,
   relish, robustness, ruggedness, secret places, sinew, sinewiness,
   small intestine, snap, snappiness, soul, spice, spiciness, spirit,
   spleen, spunk, spunkiness, stalwartness, stamina, staying power,
   sticking power, stomach, stout heart, stoutness, strength,
   strength of will, strong language, sturdiness, tang, tanginess,
   ticker, toughness, trenchancy, tripes, true grit,
   vermiform appendix, vigor, vigorousness, viscera, vitality, vitals,
   whole, works, zest, zestfulness, zip