Here's a brief summary of the available options. Also, if you just have a long lay-over and want to bump about Paris for the day, then you can consider coming into town and going to the Gare du Nord and putting your baggage in the paid short-term storage they offer: see below for details on that.

RER B regional commuter train

Roissybus direct bus service between CDG and Opera

• Pros: Inexpensive (8.60 euros), easy access (departs and arrives on street level).
• Cons: Can be slow in heavy traffic.
• Weblink: http://www.ratp.info/orienter/aeroport_roissybus.php

Les Cars Air France (bus service to Porte Maillot, Arc de Triomphe, Montparnasse, Gare de Lyon)

• Pros: Air conditioned, comfortable, reasonably priced (11-15 euros).
• Cons: Can be slow in heavy traffic.
• Weblink: http://www.cars-airfrance.com/

Taxi

• Pros: Easily accessible, takes you directly to your destination.
• Cons: Moderately expensive, between 35-45 euros, or more depending on traffic. Can be slow in heavy traffic, 3-4 passengers maximum depending on size of taxi and amount of luggage. Surcharges for 4th passenger and excess baggage.

RATP bus 350 (between CDG and Gare de l'Est) and 351 (between CDG and Nation)

• Pros: Least expensive (4.50 euros), easy access.
• Cons: Slow due to numerous stops, can be even slower in heavy traffic. As these are regular city buses, there may be little room for luggage.
• Weblink: http://www.ratp.info/orienter/aeroports.php#

Shuttle

• Pros: Reasonably priced to moderately expensive. Private shuttles have driver meet you inside airport and deliver you directly to your destination.
• Cons: Can be slow in heavy traffic. For shared shuttles, must call for pickup at airport and wait for shuttle arrival; shared shuttles can also be slow depending on the number of passengers and different drop-off points.

Limousine

• Pros: Individualized service, higher level of comfort, delivery directly to destination. Driver meets passengers inside airport.
• Cons: Can be expensive.

Motorcycle Taxi ("moto taxi" or "taxi moto")

• Pros: Saves time due to ability to get around traffic jams.
• Cons: More expensive than a regular taxi, limited space for baggage, exposure to the weather (though appropriate gear is provided).

• You can take the RER B from the airport to Gard du Nord.
• Open Monday - Sunday 6h15 - 23h15.
• Three sizes of lockers which cost 4/7/9.50 euros respectively.
• Must pass through x-ray (metal and bomb detector) to get in.
• Ticket info: DO NOT FOLD TICKET.
• There is a 20€ lost ticket fee.
• The tickets are good for ONE TIME ONLY. You open the locker ONCE to get your stuff IN
  and ONCE to get it out...and you can't go back and stuff more in for the same price. You would have
  to buy another ticket.
• The fee was listed as per locker for 48 hours, and beyond that it is 5€ per day/per item.