The most verifiable type of offset that I've heard about is landfill methane capture. You can measure the amount of methane captured, and that is what you've offset.
Typically it's burned to make CO2.
CO2 lasts longer in the atmosphere than methane.
Better yet to actually use it for something that will offset fossil fuel use though.
I think they burn the various CFCs and HFCS they extract from old refrigeration units for similar reasons.
However before it breaks down methane is a wildly more potent greenhouse gas. It still takes decades to break down, during which time it's going to be multiple magnitudes more impactful that the CO2 it will eventually become either way.