This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
This error would be caused by either - not having the jar files in the WEB-INF/lib directory - having JSTL1.0 jar files in the WEB-INF/lib directory.
Make sure - you have the correct version of JSTL installed for your container. - JSP1.2 = JSTL1.0 - JSP2.0 = JSTL1.1 - you are using the correct URI. For JSTL1.0 it is "http://java.sun.com/jstl/core" (note it doesn't include "/jsp" which was added for JSTL1.1)