With all of the caveats of benchmarking not really being able to approximate real monkeys banging on your website, you probably want to look at ApacheBench, which essentially bangs on your website in place of monkeys. ApacheBench comes with Apache. There's a web gateway here, and some "hello world" (literally!) benchmarks here to get you started.
perl -pe '"I lo*`+$^X$\"$]!$/"=~m%(.*)%s;$_=$1;y^+*`^ ve^#$&V"+@( NO CARRIER'