There are several ways to achieve this:

1. Open the server.xml of your Tomcat installation.
   
   Find the line:
   
   connector port="8080"
   
   and change it to 
   
   connector port="80"
   
   Make sure you change the HTTP Connector and not the AJP Connector. Furthermore make sure, that the user that starts the Tomcat server has the right to bind to a port below 1024. This is important on linux systems. 
2. When running Apache aside of Tomcat, you can add a virtualhost to the httpd.conf and add a AJP Connector module to it. (E.g. mod_proxy or mod_jk for Apache or isapi_redirect for IIS)
3. Use iptables for port forwarding, e.g.:
   iptables -t nat -A OUTPUT -d localhost -p tcp --dport 80 -j REDIRECT --to-ports 8080
   iptables -t nat -A OUTPUT -d your hostname -p tcp --dport 80 -j REDIRECT --to-ports 8080
   iptables -t nat -A PREROUTING -d your hostname -p tcp --dport 80 -j REDIRECT --to-ports 8080
4. Use JSVC on Linux systems. Click here for more information.