When looking up visa requirments, the Wikipedia pages for "Visa requirements for X citizens" and "Visa policy of Y" (where X is your country and Y is the country you want to visit) are quite useful. However, is there a place where consolidated information is provided about which countries allow certain other visa holders to enter without needing to get a separate visa? As an example, Romania is not currently part of the Schengen Area, but it does allow multiple-entry Schengen visa holders or residents of Schengen countries, regardless of nationality, to visit Romania for a limited time. Those who hold a valid US visa can also visit Mexico without an additional visa, for another example.
Edit: I've gotten another duplicate marking with the Schengen question, so just to clarify: this question is more generic. It's asking for information or a place to obtain information regarding countries that allow certain visa holders to enter without an additional visa. An answer could of course refer to the answer of that question for information regarding what's available to Schengen visa holders.