Netscape (from 2.0) and Microsoft's Internet Explorer (from 3.0).
Note that Netscape does not pop up a window for a tool if this window already exists. Hence, when you do not use a window of the WebKB set of tools, it's better to close it than to iconify it or to put it in background.
Sometimes Netscape do not open windows any more. In that case you have to exit Netscape and start it again.
Use the mouse middle button if you want the tool menu to appear in a different window.
The hierarchy navigation tool is slow (you should save the document which countains it on your local disk to really use it). Do not click again on an hyperlink before a new window appears, otherwise the second click will also be taken into account but for an hyperlink in the new window.
The WebKB set of tool is free software for research purpose only (cf. copyright).