[SalesForce] How to retrieve multiple contacts with single REST call

If this is a one time export or monthly export that needs to be done, you can use the Monthly Export Service built in to Salesforce. Go to Setup->Data management->Data Export and click "Export Now" or "Schedule Export" and make sure "Include images, documents, and attachments" is checked along with any other data you want to export. Salesforce will export the data, zip it up and e-mail you when the export is available for download.

The standard REST API only provides doing object describes on single objects at a time. http://www.salesforce.com/us/developer/docs/api_rest/Content/dome_sobject_basic_info.htm

You could try creating a custom REST API using Apex REST and returning the map of objects:

Map<String, Schema.SObjectType> gd = Schema.getGlobalDescribe(); 

You would have to cast the Schema.SObjectType object to a custom object first before returning it though.

Something like the following...

@RestResource(urlMapping='/RT/*')
global class RESTTest 
{
    @HttpPost
    global static Map<String, TestObj> getGlobalDescribe()
    {
        //TODO: iterate through Schema.getGlobalDescribe() method results and
        //cast results to custom object below

        return null;
    }

    global class TestObj
    {
        global String name;
        //TODO: add more properties as needed
    }
}

This code hasn't been tested by the way, is just meant to help point you in the right direction.

Check out more information on Dynamic Apex here: http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_dynamic_describe_objects_understanding.htm#apex_describe_object_access_all

Let me know if you have any questions.