In reply to Blobb:
Well the first thing is to not think of it as legitimsing your actions, otherwise it will do your head in, as per cheat neutral.
However, if you have to fly (or whatever else might emit GHGs) you can pay towards a project, somewhere else (normally the developing work) that will be reducing emissions. Theoretically, you produce x tonnes when you fly, so you will pay for the reduction of x tonnes in a project. Whether these emissions will be an absolute reduction on business as usual is another large bone of contention. Again, I take the view that it is better to get these projects up and running than not have them at all. The money you spend does facilitate that action, the revenues from selling credits being the main financing process for these projects.
Projects can take many forms from a pure renewable/fossil fuel substitute at one end e.g. a wind farm in China to handing out efficient woodfuel stoves to the poorest sections of society in Kenya.
Broadly there are two types, they can go through the official UN system, which has more stringent standards, creating credits known as CERs. Or they can be 'voluntary credits' which vary from a voluntary code of process (often under the VCS) to none at all with a company claiming they have credit 'credits' through 'emission reductions'.
Overlay this with 'the gold standard', known as GS, a 3rd party NGO run system, which requires projects to have a sustainable development component to get certified. So a wind farm in China is highly unlikely to get GS certified, a project in kenya more so.
GS projects can be UN or voluntary, but if they have GS then they will be robust. So either would do. (pick a project that takes your fancy)
So, to get to the point, yes, it's worth offsetting. No, unless you are very lazy, I wouldn't just tick the box on your airline booking form (probably large scale CDM energy, or hydro projects).
The GS and project developers can be found here
http://www.cdmgoldstandard.org/where-to-buy-gs-credits/europe
I would suggest ClimateCare (and their website) as a good starting point
http://www.climatecare.org/
Hope that's somehelp, was trying to be pragmatic rather than stir up a debate.