There are any number of possible solutions to the greenhouse effect, but the most credible ones can be boiled down to two major categories: 1) use less in the way of fossil fuels or 2) find a way to keep the carbon dioxide out of the atmosphere.
Within category one, you have three kinds of solutions: A) alternative fuels, B) more efficient technologies that don't consume as much fuel and C) just using less fuel given the technologies we have now.
I'll give one or two examples of each one:
1A) windmills and fuel made from algae
1B) electric cars
1C) just don't drive so much and turn the lights off when you leave the room.
2) Pump the carbon dioxide produced by electric power plants down into the earth where it gets trapped within the rocks.