What you can see at the Piazza Pietra site is all that is left, a part of the northern wall, of the Hadrianeum or Temple of Deified Hadrian. It was built in honor of the dead emperor by his adopted son and successor, Antoninus Pius in 145 AD. The pillars would be white if clean, but the white marble that once clad the cella wall, behind the pillars, is long gone along with the pedimentary decorations -- they represented Rome' provinces, stabilized and pacified by Hadrian, and war trophies, arms, and armor. Some of the decorations are spotted around Roman and other European museums: the best are in the Palazzo Dei Conservatori Museum on the Campidoglio and the Archeology Museum in Naples. One pillar is missing from each end of the set: all eight pillars from each end are missing along with all of the pillars from the southern side. The temple and its tall pediment stood inside of a much larger rectangular portico, which had its front entrance set back about fifty meters from the Via Lata (now Via del Corso). The eastern end of Via Recta (which ran straight [i.e., "recta"] west from the Via Lata to the Tiber) was on the northern flank of the portico. That side of the portico had a large semi-circular excedra at its center.
This is one of the few sites in Rome about which you can learn more on the Internet than at the site of the ruins, but you still need to visit the site to get an idea of the scale involved.
A picture of the model of the Temple in the Museum of Roman Civilization (in EUR, Rome) shows the southern side, the mirror image of which is visible in Piazza Pietra. It is on the Internet at http://wings.buffalo.edu/AandL/Maecenas//rome/hadrianeum/ac990813.html.
Other pictures, including some old prints are available at http://www.siba.fi/~kkoskim/rooma/pages/HADRIANE.HTM and at
http://wings.buffalo.edu/AandL/Maecenas//rome/hadrianeum/section_contents.html.
A description of the temple from the Platner and Ashby Topographical Dictionary of Rome is at http://efts.lib.uchicago.edu/cgi-bin/eos/eos_page.pl?DPI=100&callnum=DG16.P72&ident=250
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Hadrian scattered architectural marvels throughout Rome and the Empire, and for the major structures he was his own chief architect. But, with humility completely at odds with earlier and later emperors, he put his name on none of his buildings in Rome. (His name was, however, on the dedicatory plaque on the Temple of Trajan and Plotina, which he erected to honor his adoptive parents. Filial duty overtook modesty.) The Pantheon, which had been completely destroyed by fire, he rebuilt in a never-before-seen style and put the name of Agrippa, the long dead builder of the first Pantheon that stood there, over the door. He built back-to-back mirror-image temples of Venus and Roma at the end of the Forum nearest the Colosseum and fired (some say executed) his more conservative architect, Apollodorus of Damascus. His tomb, now Castel Sant'Angelo, has an internal spiral ramp that reminds some architectural analysts of Mesopotamian ziggurats. Hadrian's huge villa (Villa Adriana) in Tivoli is still an architectural wonderland that he designed to remind him of his many visits to foreign lands before and during his reign.
In the provinces and around the fringes of the empire, Hadrian aggrandized and fortified cities. His most famous construction outside of Rome is probably his wall stretching across England but he neither designed nor ever visited the wall. It represents, rather, his policy of retrenchment and stabilization, which began as soon as he inherited the imperial throne from Trajan. In fact, he quickly abandoned some of the conquests memorialized on Trajan's famous column in order to straighten the Empire's borders and make them more defensible. He was a tireless traveler who visited every province in the Empire, always inspiring loyalty among the people and personal support of the legions. Both at home and in the provinces, he was recognized as a good Emperor and a splendid administrator.
Hadrian greatest achievement was the assurance of an orderly succession after his death. His early plans were thwarted by his own longevity -- a groomed successor died before he did. Eventually he settled on a nephew, Marcus Aurelius, but he was only seventeen and too young to take over immediately. Hadrian therefore adopted Antoninus Pius, who was in his fifties and therefore not expected to live much longer, as a placeholder, but only on condition that Antoninus would immediately adopt Marcus Aurelius. The plan worked exactly as Hadrian had planned except for the fact that Antoninus lived for many more years than expected and ruled from 138 to 161 AD. Antoninus turned out to be a very good emperor, and he shared and eventually passed authority to Marcus Aurelius who was even better. (Commodus, who followed Marcus, was almost as bad as Nero, however -- so nutty that historians have sought reasons to believe he could not really have been Marcus Aurelius's real son.)
The young Christian community revered Hadrian because he specifically granted them equality before the law. Like everyone else, they could be prosecuted for criminal offenses but not persecuted simply because of their religion. The Jews, however blamed him for provoking and then brutally suppressing the Bar Kokba revolt in Judea (132-135 AD). The provocative act, a decree against physical mutilation, covered the whole Empire and was not, by any means, aimed specifically at the Jews. In the rest of the Empire, in fact, it was a protection against the whims of officials, heads of families, and slave owners who previously could lop off limbs and other body parts for many offenses. The law specifically forbade castration, but it clearly also applied to circumcision, and the Jews interpreted that as an attack on their religion. The suppression of the revolt was, in fact, as efficient and as brutal as usual when the Roman army was involved. When the more zealous Jews of Jerusalem still refused to comply, Hadrian had the city destroyed and replaced by a new Roman town, Aelia Capitolina. The leaders of the revolt were sent into exile (the Diaspora), but the vast majority of the people stayed in place and quietly conformed. Later charges that Hadrian had personally ordered atrocities while in Jerusalem do not match up with the times he was there.
And then there was the young and beautiful Antinous. Like all of the Emperors before him except Claudius, and like many after him, and, in fact, like most male Roman aristocrats, Hadrian was a bisexual. The relationship between Hadrian and Antinous followed such a common Roman (and Mediterranean) pattern that it probably would have attracted no comment whatsoever except for the fact that young Antinous died so suddenly and unexpectedly.
The great male love in Hadrian's life ended tragically when Antinous fell into the Nile River and drowned in October, 130 AD. Literally nothing more is known of the circumstances of his death, but conspiracy theorists have for centuries made much of the fact that the word that Hadrian used in his one sentence description of the event could mean "fell", "threw himself", or even "was thrown" into the River. A common assumption among conspiracy theorists is that Hadrian's wife, Sabina, had it done, but this is belied by the many descriptions of the good relations between Sabina and Antinous. Another theory says that Antinous committed suicide either to increase the next flood of the Nile or to prolong Hadrian's life: either of these would be valid in local mythology.
A temple was dedicated in honor of Antinous as was the Egyptian custom when young people drowned in the Nile -- they were thought to have become one with Osiris. It is not clear that the grief stricken Hadrian sanctioned or even initially knew about the deification, but later writers, and especially Christian writers, certainly condemned him for it. God punished them for their relationship and, in advance, for "Hadrian's" deification of Antinous.
Hadrian Internet links:
Encyclopedia Britannica biography:
http://www.britannica.com/bcom/eb/article/7/0,5716,39537+1+38745,00.html
Catholic Encyclopedia biography (one line mention of Antinous, serious inaccuracies in the very short section on Bar Kokba, otherwise accurate):
http://www.newadvent.org/cathen/07104b.htm
Hadrian links from about.com:
http://ancienthistory.about.com/homework/ancienthistory/msub_hadrian.htm
Timeline of Hadrian's career:
http://ancienthistory.about.com/homework/ancienthistory/msub_hadrian.htm
Hadrian and Antinous: