demoWebCallout.java

public with sharing class DemoWebCallout {
	
	public class applicationException extends Exception {}
	
  Public static HttpRequest buildRequest(Lead l){
		HttpRequest request = new HttpRequest();
		String url = 'http://www.pinguinshow.com/';
	  request.setEndpoint(url);
	  request.setMethod('GET');
	  return request;
  }

	public static HttpResponse makeRequest(Http h, HttpRequest request){
     HttpResponse response = h.send(request);
     return response;
	}  

	public void handleResponse(HttpResponse response){
		// For the purpose of this contrived example, we'll not do anything with the response.
		// If this were not a contrived example, you might see references to:
		// handling xml and other pointy markups languages
		// handling JSON -- if you're on Winter 12. More on that later.
	}

	@future(callout=true)
  Public static void makeWebCallout(Id id) {
  	if(id == null) return;
	  Lead[] l = [Select l.id From Lead l where l.id = :id limit 1];
	  if(!l.isEmpty() || test.isRunningTest()) {
	  	Http h = new Http(); //Create a new http object
	  	try {
	  		HttpRequest request = buildRequest(l[0]); // during a test with a bad id, this will throw an exception
		  	HttpResponse response = makeRequest(h, request);
		  } catch(Exception e){
		  	System.debug('Failed to execute callout! NO DONUT FOR YOU!! ' + l);
		  }
	  } 	
	}
}

TestDemoWebCallout.java

@isTest
private class TestDemoWebCallout {

  static testMethod void testRestfulCallout() {
  	Lead l = new Lead();
    l.Status = 'Open';
    l.LastName = 'Owen';
    l.FirstName = 'Bob';
    l.Street = '1234 Sesame Street';
    l.City = 'Houston';
    l.Company = 'Test McTesterson and Co.';
    l.State = 'Tx';
    l.PostalCode = '27384';

 		insert l;	
 		
 		Test.startTest();
 		DemoWebCallout AwesomeDemo = new DemoWebCallout();
 		HttpRequest testRequest = DemoWebCallout.buildRequest(l);
    System.assertEquals('GET', testRequest.getMethod());
    System.assertEquals('http://www.pinguinshow.com/', testRequest.getEndpoint());
		// working call
    DemoWebCallout.makeWebCallout(l.id);
    //failing call
    DemoWebCallout.makeWebCallout('00QC000000qnwbGMAQ'); //this id should not exist.
		Test.stopTest();
  }
}