Before offsetting it's important to reduce your carbon footprint as much as possible, this is the critical part of emission reductions, see the other pages under this heading for tips on cutting emissions. Offsetting should be considered where cutting carbon emissions is not possible.

Carbon offsets, like green power, come in all shades of green. Checkout Carbon Offset Watch for rankings.

Offsets range from purely voluntary offsets which are not audited (and can be sold by anyone) up to offsets that comply with a strict carbon accounting standard. Some of the existing standards are;

• Gold Standard
• Gold Standard for Voluntary Emission Reductions
• Voluntary Carbon Standard
• AGO Greenhouse Friendly
• GreenPower
• UK Code of Best Practice

A comprehensive 48 page RMIT guide on offset standards and providers is at www.global.rmit.edu.au/CarbonOffsets2007.pdf

You can purchase accredited carbon offsets from CarbonPlanet at www.carbonplanet.com

Some
ARK Resources www.ark-resources.com.au
Low Energy Supplies and Services (LESS) www.lowenergy.com.au 
Carbon Planet www.carbonplanet.com
Easy Being Green www.easybeinggreen.com.au
Climate Friendly www.climatefriendly.com

Trees v Energy Efficiency and Renewable Energy

From the above RMIT publication;
"Providers with a bio-sequestration focus (trees) have generally not adhered to recognised standards and assurance schemes, however, one stated its desire to become ‘Kyoto compliant’ and most are now seeking AGO Greenhouse Friendly certification.
Providers offering offsets from energy efficiency and renewable energy projects generally adhere to a recognised assurance scheme and present discussion about offset quality issues on their websites."