SlowHTTPTest is a highly configurable tool that simulates some Application Layer Denial of Service attacks by prolonging HTTP connections in different ways. Use it to test for DoS vulnerabilites of your web server, or just to figure out how many concurrent connections it can handle. SlowHTTPTest works on majority of Linux platforms, OS X and Cygwin - a Unix-like environment and command-line interface for Microsoft Windows.
Check out Wiki for installation and usage details.