In addition to all that if you intend to get behind the wheel of a car, plane, boat and so on you will need to carry licenses to that effect such as a driver’s possibly with International Driving Permit.
In addition to the other suggestions…
[Edit 2014 Feb]
For travel in many South American and African countries you will also need an international Certificate of Vaccination card proving that you have the required vaccinations (eg yellow fever). While you might be able to get the shots at the border it is much easier (and possibly safer) to get them at home before you travel.
I would also recommend keeping a list of your credit and debit cards together with the phone numbers to call in case you lose them.
In some countries (eg Argentina) you will want to keep proof of purchase of local currency so that you will be allowed to exchange any remaining pesos back to dollars when you leave.
Some other documents that haven’t been mentioned – some of these may fall into the ‘paranoia’ category, I’m not recommending all of these for every trip:
For contingent purposes, I always carry the following, in no particular order:
I also store encrypted electronic copies of the most important travelling documents on a cloud storage service, e.g. Dropbox.
Credit:stackoverflow.com‘