I picked up a Navigo Easy a week ago. It's a white and blue credit card sized piece of plastic with a chip in it and costs 2€. On top of that, you can add the equivalent of a carnet (10 tickets) at the same price as one, 14.90€. The card can be refilled as needed. There are machines in stations that will give you your balance.

Interestingly, Navigo is a "contactless" payment system so all you need to do is tap it on the turnstile and the gate opens. (There is no need to insert it into a slot, the same way as with the balance readers.) Every station seems to have at least one turnstile marked dedicated to Navigo.