The Roma Pass currently costs 30 euros (2012). It gives you two free admissions from a list of tourist sites, discounts to all other sites covered by the pass and includes bus, tram and subway transportation (but not travel as far as the airports). The Rome subway system is great. It will get you close to most any place you want to see.
The pass is valid for three days and expires at midnight of the third day. If you are visiting at least two sites covered by the pass and will use the public transportation part of the pass you will save money with it.
It also gives you discounts on certain tours and activities. You get a map and a guide to the attractions in the scheme.
When going to the Coliseum the entrance line may be very long. But, with the Roma Pass you get to skip this line.
There are FAQs (including where you can buy passes, eg. Fiumicino airport, all participating museums and sites) and further information at: