This was just announced. From now until October 28th every Friday night you will be able to take a guided tour of the Rome forum at night. The tours cost 20 euros for anyone over 13 and 18 euros for those age 6-12. Those under 6 can take the tour for free. You can book this online or you can call.
+39 06 399 67 700
Here is more information from the official site:
http://www.coopculture.it/en/events.cfm?id=146
This is a special event so the ticket does not include entry to the colosseum.
Donna