1. Wireless Mouse
You don't have to suffer from "TrackPad wrist's pain" on your travels with a laptop if you are used to the more ergo-friendly mouse. Pack one that is bluetooth and you will eliminate one extra cable to deal with. (don't forget extra batteries)
2. Mini USB Cable
More than once we faced the reality on having to borrow and external hard drive, only to find that we are missing the USB cable. Try to find the shortest possible for less bulk.
3. Wireless Adaptor
These days, every laptop comes with built in WiFi but if you are one of the few that lacks that technology, fear no more with this adapter. Now, for the best speed while traveling and to avoid the unexpected dead zones, get a pre paid 3G or 4G internet card and make sure it has enough balance for your trip. The 4G's available today, have speeds that challenge DSL download times.
No need to go BT (would be nice) but for privacy on your Skype conversations, or simply to listen to some cool tunes or watch a movie without bothering your airplane's seat buddies.
5. TV Tuner
It is a nice add-on to being able to flick thru your local channels while waiting for your flight, particularly if you are staying on a budget hotel room with limited choices.
6. Ethernet Cable
If the hotel room is charging extra for the WiFi, just plug the cable to the wall and enjoy internet without the fee.
7. Backup Install Disks
Basic programs to keep you going plus all the documents you may be needing on your trip. We would also recommend to use a system like DropBox to get all your important files "on the cloud".
8. Kensington Lock
If you are planing to use your computer on a public place, invest on one of these for the safety of knowing that your laptop is going to be there, once you come back from the bathroom break.
9. Old School Pen and Paper
Even that you have revolved your life around a digital backbone, is good to rely a little on old technology and resort to pen and paper when the situation demands. Nothing beats to have a pad and pen when your laptop takes a dump in the middle of a meeting and you need to take notes.
10. Important Documents
Leave them all in a separate bag; copies of your receipts, warranty info, 1-800 numbers to call and also, leave a little card inside your bag with your info, how to contact you and a little reward offer if returned safely back to you, or even better, embed that info into a temporary screen saver that you will activate during your trip. Chances are if lost, some good samaritan will return it. Also, use a picture of a cute baby for your screen saver, even if you don't have kids yet. You just increased your chances of recovery by 50%.