Euroquest
Online vendor of discount cards for Paris.
http://www.euroquestpass.com/

Got To Paris.net
Offers information about different discount cards available in Paris such as the Paris Visite and Paris Museum cards. You can even buy them online.
http://www.gotoparis.net/

Hostelling International
Find out more about the HI card here.
http://www.hihostels.com/

Inter Musees
Details of the Paris Museum Pass and how it works.
http://www.intermusees.com/wwwie/index.html
http://www.parismuseumpass.fr/

ISTC
Contains lots of information about discount cards available to students, youths and teachers.
http://www.istc.org/sisp/index.htm

La Conciergerie
Specializes in metro and museum passes and top-quality tours.
http://www.conciergerie.com/

Museums of Paris
Find out which museums in the city allow free entry with the Museum Pass.
http://www.museums-of-paris.com/

Paris Museum Pass
Free admission and no waiting at 60 museums and monuments in Paris when you buy a this card.
http://www.parismuseumpass.fr/
http://www.intermusees.com/

RATP
Information about the Paris Museum Pass, which gives you access to over 70 museums and monuments around the city.
http://www.ratp.fr/ParisVisite/Eng/index.htm

RATP
Find out more about the Paris Visite travel card, which can be used on the city's public transport network.
http://www.ratp.fr/ParisVisite/Eng/index.htm

SNCF
The website of the French national rail operator contains lots of information about the various discount cards it offers, including the Carte Senior.
http://www.sncf.com/

Tickets To...
Low cost city passes for visitors to London, Paris and other European destinations, as well as European rail passes.
https://www.ticketsto.com/