OWASP HTTP Post Tool was created to allow you to test your web applications to test availability concerns from HTTP GET and HTTP POST denial of service attacks –

Project

http://code.google.com/p/owasp-dos-http-post/downloads/list

Also checkout the Mitigating steps from slow HTTPS DDOS from ModSecurity

http://blog.spiderlabs.com/2010/11/advanced-topic-of-the-week-mitigating-slow-http-dos-attacks.html