If you're lucky enough to be in Rome between April 16 and April 25, you'll save some euros on visits to all state-run (and most city-run) museums and archaeological sites. Admission is free at the Mercati di Traiano, the Musei di Villa Torlonia, Centrale Montemartini, and other institutions participating in the 12th annual Culture Week.
There are a few notable exceptions to the free rule: the Musei Capitolini, the Museo dell'Ara Pacis, the Planetario e Museo Astronomico, and the Scuderie del Quirinale, host of the buzzed-about Caravaggio exhibit, which still goes for the regular admission price of $13.50 (€10).
The Italian Ministry of Culture has also organized free tours—in Italian only—of smaller gems such as the 16th-century Palazzo Zuccari, normally closed to the public (011-39/06-699-941), and the Villa dei Quintili (011-39/347-8235-240).
A little further afield, the ruins and frescoed homes of Ostia Antica, an ancient port city, are also free during Culture Week and make an easy day trip from Rome (011-39/06-5635-8099).