You have probably already heard of IDTGV, the website where you can purchase discounted train tickets. www.idtgv.com/en In case this is all new information, SNCF the company that runs French rail transport, attaches a second train onto some regular routes like Paris-Marseille with stops in Avignon and Aix-en-Provence, and sells the seats at discounted rates.
These seats go fast so SNCF launched OUIGO offering trips from Marne la Vallée (Eurodisney) to Lyon, Valence, Avignon, Aix-en-Provence, Marseille, Nîmes and Montpellier. The low cost trains run less often and you have to take the RER (Paris computer rail system) to get to Marne la Vallée but the low cost might be worth the extra travel time. Have a look at the website and compare. www.ventes.ouigo.com/search.aspx?culture=en-GB
Most recently the SNCF launched #TGVpop www.tgvpop.com/tutorial Sadly the website is only in French for now. The concept is a bit different. The tag line reads THE TGV THAT LEAVES THANKS TO YOUR FRIENDS. As an example TGVpop offers a train between Paris and Marseille and people can show their interest in taking that train 14 days before departure. Travellers are encouraged to share their travel plans using social media in order for more travellers to sign on. The train will not leave if it has not reached a certain number of confirmed bookings. Four days before the train is scheduled to leave each person receives an email confirming whether or not the train will be travelling and the final departure time. Tickets on confirmed trains can be purchased up until the last day at very reasonable rates. A one way Paris-Marseille costs between 25 and 35 euros.