Though many have declined to run, Cain is only the second candidate to quit the Republican primary (the other being Tim Pawlenty, who inexplicably dropped out in August after finishing third in the Iowa straw poll). The Campaign Status page has been updated accordingly. The field has now been narrowed to seven candidates.
Newt Gingrich is the chief beneficiary of Cain's withdrawal. Although Cain's decline in the polls already helped Gingrich, who received most of Cain's former supporters, actually dropping out has an impact of its own. Even a candidate who's dead in the water can still receive 5-10% of the vote. With Cain's departure, even those votes will need to go elsewhere. It's also possible that Rick Santorum could benefit from the most hardcore Tea Partiers--those who see Gingrich as too much of a RINO.
When put into historical perspective, the destruction of Cain will be regarded as a victory for the primary process. Cain was given ample opportunity to display his best qualities, but his weaknesses were also vetted. Voters had the ability to learn about Cain and discover why he should never be a general election candidate. It's always best to learn that before a candidate becomes a nominee, rather than afterward.