Gosh I feel terribly under-equipped compared to some of the above!

Whistle bag:
low whistle
fife
rauschpfeife
high whistle
F whistle
spare rauschpfeife reeds
strap for slinging the bag over my shoulder if needed

Flute case:
Flute
flute stand
cork lube
spare pads
lapping thread
jeweller's screwdriver
business cards for band & self

Concertina case:
Concertina
screwdriver
more business cards

And that's about it for acoustic gigs/sessions.

As others have mentioned, though, amplified gigs are a whole different kettle of fish, and there's probably stuff in the boxes and cases that even *I* don't know why they're there any more ...