TRIP PREPARATION & RESEARCH
U.S. Department of State: Country Info pages are the best source to use when figuring out entry/exit requirements for the country you're traveling to.
CIBT will provide assistance with obtaining travel documents and visas once you've established what your destination requires.
Transportation Security Administration is where you can apply for the TSA Pre-Check program and read tips for getting through airport security faster.
Global Entry takes a little effort (application) and time (interview) but the ability to pass faster through security upon return from abroad makes it all worthwhile. Highly recommend for frequent travelers.
Download the Mobile Passport app - the only official US Customs and Border Protection app, to speed up re-entry into the United States.
Checking the international weather forecast will help you pack a better carry-on. This Norwegian website has less annoying ads and articles than other more well-known weather websites, though you may need to change the default language to English first.
XE is the most reliable and accurate website for currency conversion.
Center for Disease Control: Travel provides credible health information. Nothing replaces the advice of your family physician, but this is the place to start researching required and recommended immunizations for travel to foreign countries.
An unlocked iPhone is a useful tool to carry on a longer trip, when it makes the most sense to buy a local SIM card to stay in touch with locals and browse the internet using data. T-Mobile now offers a plan that doesn't have any roaming charges (check the fine print to see which countries are excluded).
Hotel Tonight is an app that helps you find accommodation deals from as quickly as that same evening, to as far out as one week in advance.
TripCase is the place to store all your travel details. If you're a business traveler, this is particularly helpful to store your travel receipts.
What's one thing everyone almost always forgets to pack? Travel insurance. Even if you have medical insurance through your workplace, that doesn't necessarily mean you'll be covered while you're abroad. Start at Bootsnall's comparison chart of different travel insurance plans.