What are some good automated tools for load testing (stress testing) web applications, that do not use record and replay of HTTP network packets?

I am aware that there are numerous load testing tools on the market that record and replay HTTP network packets. But these are unsuitable for my purpose, because of this:

• The HTTP packet format changes very often in our application (e.g. when we optimize an AJAX call). We do not want to adapt all test scripts just because there is a slight change in HTTP packet format.

• Our test team shall not need to know any internals about our application to write their test scripts. A tool that replays HTTP packets, however, requires the team to know the format of HTTP requests and responses, such that they can adapt details of the replayed HTTP packets (e.g. user name).

The automated load testing tool I am looking for should be able to let the test team write "black box" test scripts such as:

• Invoke web page at URL http://... .
• First, enter XXX into text field XXX.
• Then, press button XXX.
• Wait until response has been received from web server.
• Verify that text field XXX now contains the text XXX.

The tool should be able to simulate up to several 1000 users, and it should be compatible with web applications using ASP.NET and AJAX.