I'm the co-founder of a startup which is in the beginning and growing rapidly. I want to test an stress scenario on my IIS server to help figure out the resource needed from our cloud service, like 40,000 http request per second
Whilst I don't disagree with people who have one favourite tool, such as JMeter, or ApacheBench, the problem is, from a naïve point of view, you'll run this from one server, probably in the same Availability Zone as your server, and not get an entirely accurate load test.
Have a google about for Cloud-based load testing facilities, that way, they'll use lots of different servers, from all over the world, so you'd get a more accurate representation of when your site goes viral.
JMeter can be used for this. I've not used it for web service testing myself, but I have used it to load-test a SQL mirror, as well as load-testing a Java application server.
Edit: I suppose, since I've used JMeter for load-testing a Java application server, that could be considered web service testing.