If you need a more sophisticated product, I've used jmeter before. It is distributed, supports all kinds of protocols, scenarios, etc. If you want to stress test a checkout flow it can be good. It's kind of an ugly mess though.
https://github.com/newsapps/beeswithmachineguns/
(Also, the name is awesome)
Enter something like: http://domain.com/article/$RND(1000,9999)} to get random urls every time.
Please note, that website needs to be public accessible.