Fossil fuels make up about 90% of the worlds total energy needs, the next largest provider is nuclear energy with about 6%.
Nuclear energy has the largest potential just becuase of the infustructure already in place and our knowledge of fission reactions. Fission reactions have alot of unusable radioactive products which are going to be around for about 10000 years after they are used. So expanding this as our main energy source will produce alot of unwanted, unsafe products to dispose of. The other nuclear reaction, fusion is the same reaction that occurs on the sun and has safe, usable products and it generates more energy. Fusion, however has not been preformed in the laboratory and is not close to being used by the entire world.
Wind, Solar, Tides
These are a clean, environmentally safe way to produce energy, but if mass used it would not meet the worlds present energy needs but it would be close. The downside is massive amounts of money need to be invested before energy can be harnessed, and people do not like things such as wind turbines, solar panels and beach reducing tide generators. These will suffice when the world runs out of fossil fuels, but they are not the answer.
Conventional science and conventional theory claims that energy cannot be created or destroyed and thus we have explored almost all of our energy generating potential. Unconventional scientists do believe that an alternative source of energy can be found using different theories on things such as the atom. Many of these ideas have been produced and independantly proven, but patents have not been issued becuase they do not obey the laws of conventional science. Some of these can be found at the following sites
Many of these ideas may not work, but if one of them generates more energy then it takes in the world will have energy for the rest of time.