What countries waive visa requirements for those with a USA visa?

A quick google and kudos to Vladimir Menkov's comprehensive response on Quora posted on 16 February 2016:

This may be a surprise to some readers, but quite a few countries do indeed allow visa-free entry (as a short-term visitor) to people in possession of a current US entry visa; as of 2015/2016, this includes Mexico, several countries in Central America and Caribbean (e.g. Costa Rica and Panama, as well as some island countries), and, strangely enough, Albania. Note that Canada is not one of these countries. (To visit Canada visa-free, you'd need to a be US permanent resident; however, if you have a current F or J visa in the US, it is not that difficult to obtain a Canadian visitor visa at a Canadian consulate).

Having a US non-immigrant visa in one's passport may also facilitate transiting through certain countries. For example Indian citizens (as well as citizens of a few other countries) are normally required to have a so-called "Direct Airside Transit visa" if they change planes in a UK airport, even if they don't actually pass through British immigration/customs. But this requirement is waived if the traveler is in possession of any visa for the USA, Canada, or certain other countries: Check if you need a UK visa . Canada has somewhat similar rules for citizens of some countries transiting through Canadian airports on the way to or from the USA: Transit through Canada without a visa . Hong Kong, although not a country in its own right, has a separate visa policy from the rest of China; they, too, have special provisions for person in transit to or from the USA with an appropriate visa.

The best resource for you to check the current rules for individual countries is TIMATIC, which is the info system to which most airlines subscribe. You can use a TIMATIC interface e.g. here... United Airlines. ... Enter your citizenship and the destination country, and you will get a report like this:

Mexico - Destination Visa
Visa required.

The following are exempt from holding a visa:
A max. stay of 180 days: for holders of a visa issued by the USA.

Important: Visitors not holding return/onward tickets could be refused entry.

Always pay attention to the fine print, e.g. the requirement to have a return ticket, or having previously used the US visa to enter the USA. Some airlines or points of entry will check that.

Also, the check-in staff at some smaller airlines (especially in Mexico and Central America) may not be informed about the rules like this (don't they pay for a subscription to TIMATIC? I guess they don't!), or may be rather poorly organized in general. So give yourself ample time at the check-in, in case they need to review the rules. Have a printout of the page from TIMATIC, or the info page from the country's consulate's web site.