i dont know about others, but i never travel anywhere, without a book, to read. Dont know if other people consider this essential, or not.
Even when going to warm places, i always carry a light sweater, just in case it gets cold at night, and for air conditioned places, and to wear on the plane. Would this be essential?
I would also consider some plastic bags essential, to pack clothes in, in case it rains.
FYI, we have compiled a list of things to pack (following the discussion on this thread more than two years ago). 
You can find it under Tools -> Packing List
I would add the cell phone and iPod chargers. What good are either of these items if they are dead. Batteries will not help revive them!
A small thing that is useful is a spare key chain. Especially if you are touring different areas, I find it much easier to dedicate a key chain entirely for room keys. Mine is decent size, so not to be too bulky but not easily lost. I keep mind along with my personal docs etc in my travel wallet/money belt
A padlock when staying at hostels to lock up your passport etc and maybe a chain to chain backpack on train.
Safety FIRST
I'm not sure if it's been already mentioned, but it's good to have a list of important phones and addresses, like your embassy/consulate in the country you're going to; local police number; contact info for anyone you might know/friend or family of a friend/collegue who lives in or nearby your destination, just in case you need local help.
I also always send important work-related (or other) files/documents to my e-mail account before travelling, or upload them to an online file storage. That way I have it all available in case I need some of it at my destination. Even if you have your laptop with you, this is a good way to backup stuff. And in case your, for example, external HDD falls onto a floor and dies (like it happend to my friend last year), again, you have at least some backup available.
Special tip for those attending conferences: send yourself (to e-mail) your conference presentation and/or poster file! If needed, you can always ask the organizer to print out your poster if you lose the original, and you can download your presentation if your laptop or external HDD or stick dissapear. Better be safe then sorry!
[ Edit: Edited on 06-May-2009, at 03:11 by anamarija ]
Quoting Peter
- Passport
- Tickets & Itinerary
- Toiletries (Soap, toothbrush, toothpaste, comb)
- Clean socks
- Clean underwear
- Clean change of clothes (depends on location & temp.)
- Address book (something with important contacts & emergency info)
- Currency (Travellers cheques, cash, credit)
- Backpack
- Pen/Pencil
- Camera/FilmI think that covers the VERY basic packing list.. please add anything you think absolutely vital to all travellers (or suggest that I take something off and tell me what it depends on).
I would add tissues and some medicine