ApacheBench is a command line computer program for measuring the performance of HTTP web servers, in particular the Apache HTTP Server. It was designed to give an idea of the performance that a given Apache installation can provide. In particular, it shows how many requests per second the server is capable of serving.
The ab tool comes bundled with the standard Apache source distribution, and like the Apache web server itself, is free, open source software and distributed under the terms of the Apache License.
More Info : Performance Benchmarks a Webserver