There are 19 countries that are not part of the Schengen Area and which you can visit without a national visa if you do hold a schengen visa.
For more information, please have a look at
http://www.schengenvisainfo.com/non-eu-countries-where-you-can-go-with-schengen-visa/
Since each of these countries would unilaterally decide whether they recognize Schengen visas and under which conditions, I don’t think it’s possible to find an official list anywhere. I am marking this answer as “community wiki” so that we can come up with a list:
According to Wikipedia, Schengen visa holders can visit Serbia for 90 days. The Serbian Ministry of foreign affairs confirms that information.
Montenegro – According to Visit Montenegro holders of Schengen visas may enter, stay in or cross the territory of Monenegro for up to 30 days (but no longer than the expiry of the visa, if the visa expires in less than 30 days).
Credit:stackoverflow.com‘