Is it possible to counter WebBrowser memory leak by using it in separate AppDomain and reloading it once in a while to reset used memory?
The WebBrowser leaks you're talking about most likely happen in the underlying ActiveX control, which is unmanaged code. AppDomain helps isolate managed code only so cannot be of much help fighting down the leaks under question.
You would have to resort to running the WebBrowser in a separate process should you really suffer such leaks, I'm afraid.