It used to be just a guitar. Then things got increasingly complicated...Now my checklist for a gig includes:

2 folding chairs
CDs to sell
Mikes, patch cords to plug into our compact sound system
Boom mike stand
Guitar stand
Music stand
Song lyrics and fiddle tune binder--used to remember all the lyrics and tunes--now I need prompters...
Clothes pegs--for outdoor gigs like street markets--to pin down pages of song binder from gusts of wind
Seed money for guitar case and change for CD purchasers
Replacement strings, kapos
oh--and a guitar