Has changed with every job I've had, but currently:
Sony PDA
Mini Maglight
Small note pad
Wallet with currency of whatever country we are going to...left over from previous trip's allowances.
Several pens...all free from hotels etc
One Sharpie marker
Small digi camera
Sanicom wipes
Wet and Dry CRT/LCD screen wipes
Mouldable foam earplugs
Sennheiser Noise reducing headphones and i-pod shuffle
2 School type exercise books with type rating notes and line training/flying notes
World Atlas
Current and previous roster
Crew rest bag (toothbrush and mini paste, panadol, lip balme, eyeshade, sleeping t-shirt etc)
Small laptop (13in)
Company manual suite on CD
Cellphone
Carkeys
Folder with licence and passport
In general, the further you go along in your career, the less stuff you tend to carry.....possibly because you get sick of carrying a bunch of stuff you never use. I flew with a long haul Captain the other day who carried nothing but a small sized paper shopping bag....legend!