The Palazzo Massimo is one of several buildings, also including Palazzo Altemps, the Crypta Balbo, and Museum of Musical Instruments (near the basilica of Santa Croce in Gerusalemme), and a recently reopened section of the baths, that now hold parts of the national collections that formerly were all crammed into the Baths. The national collections, founded after Italian unification to consolidate nationalized and purchased treasures, simply grew too much to be contained in the Baths, which were also desperately in need of repair. This led to the decision to split the collections among the various sites. They, together with, constitute the Museo Nazionale Romano.
Palazzo Massimo holds Ancient Roman art (sculpture and paintings), coins and jewelry distributed over three floors. In the basement are the numismatic and jewelry collections. The numismatic collection, now rated as the premier Roman coin collection in the world, was assembled from several formerly private collections (those of the Museo Kircheriano, Francesco Gnecchi, and King Victor Emanuel III of Italy) and from the most important coin hoards discovered in Rome and the surrounding Latium region. The exhibit illustrates the story of Roman money, from its origins to its function in modern times, describing the complicated social, political and economic aspects of coinage. There is an additional, even more extensive collection available for study by scholars.
Adjoining the vault containing the coin collection are display cases for jewelry discovered in ancient burial grounds in Rome and its suburbs, which illustrate the history and evolution of fashion and costume in the Roman Empire.
The collection of ancient art, covering the ground, first, and second floors of the palace, include many celebrated examples of Roman art dating from the late Republican period to the end of the Roman Empire, as well as several original Greek works discovered during excavations in the Gardens of Sallust.
On the ground floor, a rich display of portrait sculpture, enhanced by mosaics, inscriptions and decorative sculptures, documents two eras which revolutionized Roman society, the first occurring after the conquest of Greece and the second during the transformation of the Roman State from Republic to a great Mediterranean empire. Also demonstrated are the two competing styles of sculptural portraiture, the more naturalistic style, which was favored by the "popular" party, and the more idealized classical Greek style, which was more appealing to the "optimati", who considered themselves to be of a higher social class. (The distinction between "populares" and "optimati" was really ideological rather than economic -- "populares" could be rich and "optimati" poor -- but that's another story.)
The sculpture collection continues on the first floor with more portrait sculpture and a large hall containing a number of important and famous pieces of Roman ornamental statuary. Also on this floor are bronze ornaments from the Emperor Caligula's great pleasure barges excavated from Lake Nemi in the early 20th century. The second floor has an impressive collection of mosaics and pictures from ancient villas in and around Rome. These include especially fine frescoes and stucco designs from a Roman villa found on the grounds of the villa Farnesina on the via Lungara in Trastevere, which are very complete examples of the refined and classical taste of the Augustan Age (early first century AD). Another outstanding work in the collection is the barrel-vaulted chamber containing the frescoes from an underground room of Livia's villa at Prima Porta, which are among the best conserved illustrations of an ancient Roman garden.
The Blue Guide City Guide for Rome (seventh edition, 2000) has a short description of the building and of the collections with floor plans and you will need this or one of the Museum's own more detailed guidebooks, because signage in the Museum is not really adequate. Audioguides are available in English and other languages. The Museum is closed on Sunday afternoons and Mondays. It is accessible to the handicapped. As you might expect, the museum has one of the better bookstores in Rome and offers temptations that are hard to resist (major credit cards accepted.)
The official Palazzo Massimo site has the most information, but only in Italian: http://www.archeorm.arti.beniculturali.it/sar2000/museo_romano/Pal_massimo.htm
Italy cyberguide: http://www.italycyberguide.com/Geography/cities/rome2000/F18.htm
Links to all parts of the branches of the Museo Nazionale Romano (Italian only): http://www.archeorm.arti.beniculturali.it/sar2000/Museo_romano/mus_naz_romano.htm
For a timeline on the urban development of the area of Rome around the Museum, go to: http://www.mmdtkw.org/VPiazzaTermini.html
P.S.: The only known Roman mummy, that of an eight year old girl found in a tomb on the Via Cassia, is on display in a room near the coin collection in the basement. It is completely unwrapped and not for the squeamish. Another "Roman mummy" on display at the church of Santa Fortunata in Moquegue, Peru, is not a real mummy but actually is a wax effigy containing the skeleton of a 17 year old woman from fifth century Rome. It was "translated" from the Roman catacombs to Peru in the late 18th century. "Roman period" Mummies continued to be made in Egypt during the several centuries of Roman domination, but they were Egyptian rather than Roman.