Public Transport: Trains, Bus, Metro

RATP - Paris Public Transport Network
The RATP operates most public transport in the Paris region. Their website offers a lot of useful information about tickets and travelling in the Ile de France as well as traffic conditions.
Eurostar offers several fast trains a day between London St Pancras International and Paris Gare de Nord. Direct services between London and Ashford to Disneyland Paris and (in season) to the French Alps.
Noctilien: Night buses
Noctilien: more night bus lines for the Ile-de-France region! Night travellers, the new regional bus network is just for you! The Noctilien network means more buses, more frequently (35 lines throughout Ile-de-France) See website for details.
Bus service that serves 1500 different destinations in Europe, discounted tickets available if you book at least one month in advance.
Thalys operates high speed trains between Paris and Northern European cities. Their site offers online reservations and information about destination cities.
BMS Voyages
British Marketing Services is BritRail Paris Office, all railways tickets and passes to UK and Ireland. Access to Point to Point tickets and passes, fares available only to non-UK residents. At 64 blvd de Sebastopol (Arr 3) Metro: Reamur Sebastopol
SNCF - Transilien
The Transilein is the SNCF network serving the Paris suburbs. Most of the information you need to know is best found through the RATP.