Spare (charged) battery (and/or your air-craft seat-power (it's called "Empower") adapter _ but beware that even today the planes that have this (in your class of travel) are very rare - my best successes here are Cathay, Singapore trans-pac and to-from Europe on Boeing aircraft.  The Asian carriers don't seem to have the "Empower" seat-power feature on Airbus aircraft - the key here is to ALWAYS ASK before you fly... 

2-4 floppies and/or 2-4 CD-ROM (recordable) : with your main / key / necessary presentations, files, spreadsheets on them - in case you arrive at your destination and your machine's dead (this is a WHEN, not IF thing)

Overhead (plastic) slides: of your presentation-slides

Send-Ahead: your files (same ones on the floppies / CD-ROMs ) via e-mail or put them on the web, somewhere - so you have backups of your backups in case your machine gets hosed somewhere along the way.

Spares: AC-power adapter - one of the reason(s) you take the above noted spare battery is that if you arrive and your destination and your existing battery is dead, and the AC power adapter is dead, you'll need a way to power your machine until your can get it re-charged.  Interestingly enough Dell has a fairly poor record.  Many of their AC-power adapters fail within the first 12 months.  Having a "12 month, Global Warranty" won't help you a heck of a lot in mid-western China on a weekend, take an extra AC-Power adapter.

spare AC power adapter AC cord :  In the "olden days of yore" our laptop computers used the same basic power cord that the desktop computers did, with the 3-pin grounded socket on the end that connected to our AC-power adapter.  However, today the laptops & their adapters are smaller and lighter and most laptops use the power cord with a small "8" shaped 2-pin socket on the end that attaches to the AC-power adapter. (I know this one works with Toshiba and Apples)   VERY FEW sites have a power cord like this to loan you, or that you can borrow.   If you can't charge your laptop - it's pretty useless. - so take a spare AC-power-cord... 

AC power adapter local-wall-socket adapter:  There's another page in this series Electrical Outlet Specs that helps you determine what the power socket will look like in the country where you'll go.  There's also a '...gif' file of a small card that comes with adapters made by a company called "Wonpro" (Taiwan).   These, and/or other brands are available in almost all airport shops now.   Make sure YOU BUY YOUR OWN, don't depend on the site where you're going to supply you with something.  That's not their job.  And though 'some' hotels may be able to loan you one, again, don't count on that... 

Modem and/or Ethernet cables if they are specific to your laptop and/or PCMCIA cards (that is - they have special connectors (which is very VERY common)).   Remember - you bought your laptop in your home country with your local vendor.  This combination of laptop and/or PCMCIA cards (and their funny little connectors) may not be readily available in the country you are visiting.    Do not plan on imposing on local staff to try to source something for your unique laptop / PCMCIA combination, they have other roles & customers to support. Trying to source your unique part is not their role or responsibility.  It is your responsibility to be self-sufficient and to come prepared for their environment.

What to make sure (at least for me) to leave at home:

External drives: Leave them, weight & bulk are costly, drives have a tendency to trash themselves in suitcases in luggage holds, taxis, buses, etc.   If you're going to be at a company site, by definition you'll be able to plug into their network, which means you can either e-mail (on the LAN (not across the WAN)) to your local contact,  or ftp to somewhere else, your key files.