The two solution already mentioned (DWDM & single-core-SFPs) are both methods of getting multiple signals down a single strand of fiber.
As Haakon said, single-core-SFPs work in pairs and just replaces your normal SFP/GBIC modules in standard network equipment. And as cavver said, DWDM is another method. DWDM though is more complex, requires separate pieces of equipment and is much more expensive.
http://en.wikipedia.org/wiki/Wavelength-division_multiplexing
The advantages are that you can get more data down the same infrastructure allow you to double (at least) your bandwidth. [DWDM gear can carry multiple signals (160+ channels) over a single piece of fiber and can intermix data and voice traffic.]
The disadvantages are you may sacrifice the distance the signal can travel and of course you increase your risk. If a single strand is cut, twice as much is impacted.