In no particular order apart from the first two which are very important!

1. A sense of humour
2. An ability not to get jetlag
3. A mobile phone with charged batteries that works wherever you find yourself
4. A method of backing up the numbers stored on your phone
5. A lightweight computing device capable of connecting to the internet via wi-fi, your mobile phone, ethernet and dial-up
6. A wi-fi account such as Oingoboingo or T-Mobile
7. Secure remote access to your email
8. Bottled water on your flights
9. Good quality luggage that is easy to move around and easily recognisable
10. Noise reducing headphones
11. Socks for in flight so you can take your shoes off
12. Spare batteries
13. Any electrical adapters, chargers and voltage convertors you need to get your kit working
14. Telephone and modem extension cords
15. Source of music (CD/MP3/Tape etc)
16. Contact and reservation details for the airline, hotels and car rental firms on good old fashioned paper
17. Any medication you need to take with you should be in your carryon luggage
18. Spare Ziplock bags - useful for all sorts of things including collating receipts
19. Small Flashlight that has working batteries!
20. Passport and driving licence
21. Do some research in advance of a visit to a new place and learn where the good restaurants are and what there is to see and do
22. Map of where you are going
23. Postage stamps, envelopes, writing paper and a working pen with refills
24. A portable travel plug as hotel sinks and baths often fail to hold water
25. A copy of the book "Stuck at the Airport"
26. Needle and thread for emergency repairs.
27. Safety pins for quicker emergency repairs
28. Small travel first aid kit with extra band-aids for a different sort of emergency repairs
29. Small notepad
30. The name of the place you've agreed to meet three other FlyerTalkers at for dinner and directions how to get there
31. Aircraft air can be very low humidity so take some hand cream and some lip balm
32. Take plenty of tissues
33. Disposable hand wipes are useful too

No doubt I'll add to this list as I go along but that's what I can think of immediately