Neosporin
Bagbalm (not an emergency item but nice to have on hand - does great job rejuvenating scaly legs that are not looking good for whatever reason)
V&E
Avia Charge 2000 (not a first aid thing really, but a staple)
Pure aspirin (FIVE 325 mg aspirins crushed per gallon of water for pain, when bird not bleeding significantly)
Betadine (maybe...used a few times 9 parts water 1 part betadine to clean wounds)
Antibiotics -Someone else can advise as to the best antibiotics. My feed store carries only Terramycin and Sulmet.
Clean syringes/droppers -in case a med needs to be slowly, carefully dribbled along beak line so they swallow it
Vet RX -I have this stuff somewhere, haven't had much occasion to use it and still not sure best circumstances for it or best way to administer, tho it is often mentioned on BYC
Wormer - in case get hit hard and fast with worms and don't have easy access to the stuff at nearby stores.
Something to protect combs in frigid weather
I'm sure I'm missing lots of things but this is the first bunch of things that popped into my head......
JJ
Edited to add that a number of homeopathics could be good to have on hand..Rescue Remedy may help with calming, for example.