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.
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.