What can I use to do load testing or functional testing of my web server?

Here are some links to open source web server load and functional testing programs or frameworks: 
 ab – Apache HTTP server benchmarking tool 
 
 Somes with Apache on some platform. 	 Very easy to use. Supports parameters such as concurrency and POST data. 	 It tells you how many requests succeeded and how many failed (by response code and/or timeout), the request rate, etc. However, you cannot see which ones failed and why, etc. 	 No support for distributing work across machines. 
 
 Apache JMeter 
 
 Constructs and runs test plans in its GUI . I think you can use the JMeter classes (in Java) directly, instead of using the GUI . Documentation is scarce in this area too. 	 Supports distributing work across machines. Multiple machines can load the test and another machine can observe the test. 	 Pretty limited in terms of running dynamic test cases: There are built-in functions (like one that gets the current time), regular expression, and JEXL , but you can’t use custom code in test cases easily. 	 Sends and receives HTTP requests and responses. Not aware of browser behavior or client-side scripting. 
 
 Selenium 
 
 3 variants:	 
 Selenium: write test plans in a language called Selenese in the Selenium IDE 
 		 Selenium RC: incorporate test plans into programs; server part is in Java but client drivers exist for Java, .Net, Perl, PHP , Python and Ruby at least. 		 Selenium Grid: a version of RC that enables spreading of test load among multiple processes and/or machines 	 
 	 Functional testing (all) and load testing (RC, Grid) 	 Runs in actual browsers so it can be used to test browser behavior or client-side scripting. For example, you can tell the script to click on a specific element in the page, even if the element is created by JavaScript loaded at run-time. 	 
 FAQ of Grid states that it’s not designed for load testing.	 
 One of the reasons: it’s resource-intensive to run multiple simultaneous instances of actual browsers. 		 Grid’s “principle author” stated that each machine should be able to run 10-20 browser instances. This is still much more resource-intensive than many alternatives. I’ve read somewhere that BrowseMob provides performance-testing service using exactly this software though. 	 
 
 
 HtmlUnit 
 
 A Java library providing building blocks for testing. I’ve read that front-ends exist. 	 Primarily for functional testing, but also used by many for load testing. 	 Claims to “understand” page content and runs JavaScript including AJAX . 
 
 Simple Test 
 
 Unit testing for PHP 
 	 A browser class that simulates a browser’s behavior is included 	 No real support for load testing beyond timing a request, as there is no concurrency (within the program at least). 
 
 Other 
 
 Canoo 	 List of 23 Open Source Performance Testing Tools