Since 1st October 2011, La Poste (the French post office) is selling green stamps (timbres verts). Instead of taking 1 day, the letter will take 2 days to reach its destination. It can only be used within France.
This green stamp is more eco-friendly as it will travel by train rather than plane.
It is €0.57 instead of €0.60 (normal post). I have recently started to use the new stamps for sending letters.
I will be saving some money (not a lot!) and the planet at the same time!