Hidden features of JSP/Servlet

Question

I am interested in your tricks etc used when writing JSP/Servlet. I will start:

I somewhat recently found out how you can include the output of one JSP tag in an attribute of another tag:

<c:forEach items="${items}">
  <jsp:attribute name="var">
    <mytag:doesSomething/>
  </jsp:attribute>
  <jsp:body>
    <%-- when using jsp:attribute the body must be in this tag --%>
  </jsp:body>
</c:forEach>