ATAC
Official source of information about Rome's metro system; the site includes maps and information about fares, lines and timetables.
http://www.atac.roma.it/

Metro Roma.it
Includes maps, line information and timetables.
http://www.metroroma.it/

Roma Matic
Interactive map of the Rome metro system.
http://www.roma-o-matic.com/metro_en.php3

Rome Buddy
A map of Rome's metro system.
http://www.romebuddy.com/givesadvice/subwayroutes.html