Lifter & striker(aka transfer bar) & spring, firing pin and spring, and a hammer spring. You can keep a spare hammer if you wish, but the only time I've seen em reported broken is when someone is doing a "McPherson trigger job" by pushing on the hammer and pulling the trigger. I don't remember a broken hammer spring, but some weak ones once in a while.

Of course none of these parts will do you any good if you don't have the tools to R&R the trigger group, those would be covered in the FAQs in the trigger hone info.
Tim