Testing HTTP Callouts by Implementing the HttpCalloutMock Interface : http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_classes_restful_http_testing_httpcalloutmock.htm

01_CalloutClass.cls

public class CalloutClass {
    
    public static HttpResponse getInfoFromExternalService() {
        HttpRequest req = new HttpRequest();
        req.setEndpoint('http://api.salesforce.com/foo/bar');
        req.setMethod('GET');
        Http h = new Http();
        HttpResponse res = h.send(req);
        return res;
    }
}

02_MockHttpResponseGenerator.cls

@isTest
global class MockHttpResponseGenerator implements HttpCalloutMock {
    
    global HTTPResponse respond(HTTPRequest req) {
        
        System.assertEquals('http://api.salesforce.com/foo/bar', req.getEndpoint());
        System.assertEquals('GET', req.getMethod());
        
        HttpResponse res = new HttpResponse();
        res.setHeader('Content-Type', 'application/json');
        res.setBody('{"foo":"bar"}');
        res.setStatusCode(200);
        return res;
    }
}

03_CalloutClassTest.cls

@isTest
private class CalloutClassTest {
     @isTest static void testCallout() {
         
        Test.setMock(HttpCalloutMock.class, new MockHttpResponseGenerator());
        
        HttpResponse res = CalloutClass.getInfoFromExternalService();
        
        String contentType = res.getHeader('Content-Type');
        System.assert(contentType == 'application/json');
        
        String actualValue = res.getBody();
        String expectedValue = '{"foo":"bar"}';
        
        System.assertEquals(actualValue, expectedValue);
        System.assertEquals(200, res.getStatusCode());
    }
}