Sulphur dioxide (SO2) screen to reflect the sun and cool earth.
Five clusters of 3 specially developed hose pipes 2" dia (5cm) around the world would carry liquified SO2 eighteen miles (19km) to be sprayed as fine mist into the Stratosphere. These clouds will circulate within 10 days and tend to spiral towards he arctic or antarctic. The hoses would be supported by helium baloons at 100-300 metre intervals; booster pumps at 100 metre intervals would be included. The Albasca oil sands (Alberta) is one site where the rejected sulphur could be burnt to SO2 and liqufied.

In 1991 Mt Pinotubo, in SE Asia erupted and put a million tonnes of SO2 into the atmosphere at high altitude for 2 years. The average world temperature dropped by 0.6 deg C average.
200 million tons of SO2 are released into the atmosphere each year, 25% from Volcanoes, 25% vehicles and coal fied power stations the rest from natural sources (sea spray). A further 100 million tons each year injected is thought will reverse warming of Arctic.
Alternatives of fitting sulphur dispensers into the worlds airliners or by using rockets have been suggested.
A disadvantage could be acid rain and damage to the ozone layer.
Cooling ideas alone do not solve the problem of CO2 build up nor of course the limit of supply of fossil fuels. Read more.
Synthetic trees to extract CO2 from the air (recommended by the Institution of Mechanical Engineers (UK.)
100 foot synthetic trees, to suck CO2 from the air. Using sodium hydroxide to convert CO2 to Sodium Carbonate, which would be stored deep underground.

One tree would extract 3650 tonnes of CO2 per year (one real oak tree might process 0.03 tonnes a year.) Therefore 5 million synthetic trees would capture 18 Giga tonnes of CO2 (5 Giga tonnes of carbon) a year, approaching the worlds CO2 emissions. These could be located near to source of emissions, industry, motorways etc. Costs could be £12,000 each ($20,000 each.) Professor Klaus Lackner of Columbia University New York has been testing prototypes.
100,000 synthetic trees could fit into 600 hectares (1500 acres) and clean up CO2 emitted in the UK
Comment. The energy used in operation could be prohibitively high.
White and soft colours on roofs, paved surfaces to reflect the sun.
Proposed at the Climate change conference May 2009, all flat roofs should be painted white, sloping roofs in 'cool colours' to reflect heat. Paved surfaces, side walks, roads should be finished in cool concrete colours not black or dark grey colour. This would reflect heat and sunlight instead of absorbing it.
Professor Chu calculates that if this is done the equivalent CO2 saved would be 44 billion tonnes, the same as reducing carbon emissions from all cars over 11 years.
Increasing Plankton
Where in areas of the sea too warm for plankton to survive, colder water rich in nutrients from the depths are churned up, plankton and life become abundant. The growth of plankton absorbs CO2 from the sea, trapping the carbon as it dies and falls to the ocean floor. The potential in southern oceans would be to remove 1 billion tonnes of carbon a year (out of 8 billion tonnes a year man made. This would reduce the balance from 5 billion to 4 billion put into the atmosphere - see global page.)
Plankton growth emits dimethyl sulphide gas which creates cloud formation which in turn reflect the suns rays, cooling the earth.
Pumping nutrient rich water from 200m depth (650 feet) to decrease CO2 in atmosphere.
An idea to pump up water from 200m depth has been suggested by Professor Lovelock and Chris Rapley by installing 200m (650 foot) pipes 10m in diameter, buoyed up at one end, with a non return valve at the other, which with wave action would pump up say 5 tonnes of water a second.
Proposed locations of these hundreds of thousands of giant pipes are Gulf of Mexico, Christmas Island(NE australia), the great barrier reef, Atoll Islands (Indian Ocean).
Powdered Iron to be poured into Pacific to encourage Plankton growth and decrease CO2 in atmosphere.
Planktos hopes to increase phytoplankton in the Pacific by 60 million tons so as to absorb CO2 and in 2 years take 3-5 million tons (7ppm) of carbon into the sea bed to be locked up.
The sea area north of Crozet Islands (due east of the Cape, South Africa) has 260 tonnes of iron scoured from volcanic rocks by currents each year which promotes plankton blooms. Iron was dropped south of the island, plankton bloomed and carbon was taken and locked into to the sea floor when the plankton died: but the results of this experiment were so far inconclusive as far as a way of significantly reducing the level of CO2. (Nature magazine Jan - Feb 08)
Breeding Algae with high CO2 absorption rate
By selecting out algae with genomes linked to high CO2 absorption, colonies could be developed to increase the rate of CO2 draw down.
Giant sunshade in space to reflect the sun
To divert 2% of suns rays from earth a 100,000 mile wide shade, 1 million miles from earth, sitting in zero gravity consisting of 16 trillion 2 foot dia. glass mirrors 3 microns thick has been suggested. The total weight would be 20 million tons. the cost should be a few trillion dollars less than 0.5% of world GDP.

Comment. Likely to be too expensive. Does not tackle CO2 the source of the problem and could upset weather systems.
Cloud making yachts in southern ocean to reflect the sun
The yachts would atomize sea water and spray the vapor into the atmosphere which would reflect light. 1000 vessels putting 5 tonnes water a second would reflect 1 to 2% of light. The vessels would be powered by vertical cylinders that rotate with the wind. They would be positioned off Greenland in summer and repositioned in the southern ocean in the southern summer and others in the Pacific.

Cloud formation, cosmic rays and CERN
Cosmic rays are believed to encourage cloud formation which could reflect sunlight. One of the many possible benefits of the Large Hadron Collider experiments at CERN might be to research this.
Comment
One would expect that expense and feasibility need more detail assessment and it does not address the problem of oil etc running out. The more sensible approach would seem to be to reduce CO2 by green methods of generation and car propulsion, including nuclear, and cease destroying rain forest. More detail on Professor Lovelocks blog.