USA Dollars work, the world trust U.S. Dollars, if in doubt carry Dollars, I have never been to a country where the Euro was wanted more than Dollars. I purchased 1000 Euros one time in Thailand to take with me to West Africa, they wanted Dollars.
Traveler Checks are almost always worthless and dangerous to carry.
ATM or Bank Card is what almost always works, at least in large cities.
Credit Cards are only going to work in Five Star Hotel, and if you are in an Five Star Hotel, then it would not be Adventure Travel.
Countries I needed Cash?
1. Iraq in 2003
2. Cote d'Ivoire in 2011 - Suspected, not sure
3. East Timor during a Military problem
4. Amazon River - Leaving the guidebook path
I carry cash and live as cheap as possible in these countries, I want to guarantee I have enough money to enter and leave, many people leave a country because they run out of money.
Note: Sometimes when all else fails, Western Union is still working.
Sign up with Western Union and be ready to wire money, if nothing else, you can maybe wire money to a friend and the friend gives you the money. Western Union is about the only way that money is always possible, but then again, not always.
Note: I had my father bank card, I established a Western Union account in his name, then used Western Union to wire myself money. You are not allowed to wire from your own online account to yourself.
Western Union can call and ask if you did the transaction, I have a Skype.com number that forwards to where I am at, anywhere... to my local SIM card cell phone.