If you want to conduct a stress test over internet to your web site, you can try this service -
Load Impact [loadimpact.com].
The FREE test does not reflect the real situation of your web site but it is a good start to know your web site performance.
When you registered to the service, you will grant some free credits for the test. This test will be more realistic. However, it is quite expensive to purchase the credits.
Samiux