Carbon offsets have been a contentious subject on this and other environmental blogs. On one side, people argue that their sale produces better outcomes than would otherwise arise, since people voluntarily help to eliminate emissions where it is cheapest to do so. On the other, people argue that many offsets are of dubious quality, and that the very idea of offsetting perpetuates harmful behaviours and the false sense that climate change can be addressed without lifestyle changes. Not everyone can offset, after all.
In response to the former concern, about the quality of offsets, the Pembina Institute and David Suzuki Foundation has produced a survey of 20 Canadian vendors of offsets. According to Pembina, offsets from renewable energy and energy-efficiency projects are the most credible sort available. Others have pointed out that forestry-based offsets and those based on Kyoto Protocol CDM credits are among the most dubious.
In the end, I think buying offsets is a much less worthwhile exercise than reducing your own emissions or lobbying for political action on climate change. That being said, if there is going to be a market in offsets, it is good that the various firms providing them are being subjected to outside scrutiny.