A more up-to-date answer:

#
# Wide-open CORS config for nginx
#
location / {
     if ($request_method = 'OPTIONS') {
        add_header 'Access-Control-Allow-Origin' '*';
        #
        # Om nom nom cookies
        #
        add_header 'Access-Control-Allow-Credentials' 'true';
        add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
        #
        # Custom headers and headers various browsers *should* be OK with but aren't
        #
        add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';
        #
        # Tell client that this pre-flight info is valid for 20 days
        #
        add_header 'Access-Control-Max-Age' 1728000;
        add_header 'Content-Type' 'text/plain charset=UTF-8';
        add_header 'Content-Length' 0;
        return 204;
     }
     if ($request_method = 'POST') {
        add_header 'Access-Control-Allow-Origin' '*';
        add_header 'Access-Control-Allow-Credentials' 'true';
        add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
        add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';
     }
     if ($request_method = 'GET') {
        add_header 'Access-Control-Allow-Origin' '*';
        add_header 'Access-Control-Allow-Credentials' 'true';
        add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
        add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';
     }
}

source: https://michielkalkman.com/snippets/nginx-cors-open-configuration.html

You may also wish to add Access-Control-Expose-Headers (in the same format as Access-Control-Allow-Headers) in order to expose your custom and/or ‘non-simple’ headers to ajax requests.

Access-Control-Expose-Headers (optional) - The XMLHttpRequest 2 object has a 
getResponseHeader() method that returns the value of a particular response 
header. During a CORS request, the getResponseHeader() method can only access 
simple response headers. Simple response headers are defined as follows:

    Cache-Control
    Content-Language
    Content-Type
    Expires
    Last-Modified
    Pragma
 If you want clients to be able to access other headers, you have to use the
 Access-Control-Expose-Headers header. The value of this header is a comma-
 delimited list of response headers you want to expose to the client.

http://www.html5rocks.com/en/tutorials/cors/

Configs for other web servers http://enable-cors.org/server.html

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