Sendgrid Labs has a great tool for stress testing sites and applications called Loader.io.
To get started all you need to do is upload a small text file with a unique key from Loader.io into the root of your site to verify ownership, then start running your tests. In the free version you can hit the application or site with up to 10,000 clients per for a minute at a time.
The free version shouldn’t be used as the end-all test for stress testing an enterprise level site, but it’s a convenient, easy to use solution for finding bottlenecks and glaring issues.