This by default will be javax.servlet.http.HttpServlet, though I guess it is possible for different servlet containers to extend something else by default like their own custom extension to the HttpServlet.
Tomcat 5.x uses jasper2, and looking in the org.apache.jasper.compiler.Generator class in the tomcat sources, jasper builds JSP pages into servlets that extend the HttpServlet by default.
Though there is the JSP page directive
that can be used to tell the JSP compiler to build Servlets using the base class that you specify. This is handy if you have some framework or custom stuff you want to be available for all jsp pages.