An Australian expression - if you think something is awesome, brilliant etc. then it is said to be the duck's guts.
Synonyms (see below) include the bee's knees, the cat's pyjamas, or, strangely enough, the cat's arse.
My new cricket bat is the duck's guts.
Look at that pony-tailed wanker in his BMW. He thinks he's the ducks guts.