There seems to be a lot of stress/load testing tool that support AJAX.
I am wondering how well does these tool implemented.
Do they only record http request and repl
If you are worried about functionality only, something like Watin, Watir, Selenium or any functional tool for that matter would work. As long as you put sufficient timing in your functional tests to allow for ajax callbacks then that should do the trick.
To add to my response,
If you are talking about unit testing your javascript you could use something like qunit as described by a fellow LosTechian in this posting. This is a pretty inventive use of an nunit addin and qunit for integrating js unit tests.
By pure coincidence, the same person that posted that qunit testing blog post just posted one yesterday about this very topic