Clothes dryers should vent outdoors.
However, clothes dryers do make excellent humidifiers and if you really want a ton of water vapour in the laundry room you could very well vent yours indoors.
Special lint traps and by-pass valves exist to aid you in venting a clothes dryer indoors in the winter and outdoors in the summer. These will save on heat losses and humidify the laundry room. This effectively recycles the moisture back through the clothes, taking longer to dry and tends to nullify any energy savings you thought you were making. If your dryer does not vent outdoors, it should at least vent into another room. Even then you must realize that much of the humidity will end up inside your walls and little will be adequately distributed through the house. Trying to run this highly concentrated humid air through the furnace ducting system could cause rust problems within the duct and definite corrosion on the furnace heat exchanger.
Publication and manufactures that recommend venting clothes dryers indoors in the winter assume that your house is not well sealed (too dry) and ignore both the potential damage such high concentrations of humidity could cause inside the cold walls and attic and the poor distribution of the humidity you gain.
Several manufacturers of air-to-air heat exchangers provide equipment to allow you to run clothes dryer exhaust through their systems -- despite the fact that this procedure is not approved by the Canadian Standards Association. They add on an extra lint trap, which with regular maintenance will hopefully minimize extra cleaning of the heat exchanger. Then they add backdraft dampers into the household exhaust lines to prevent the dryer fan from pumping damp air up into the kitchen or bathroom. With all of this extra expense and necessary regular maintenance, you will recuperate about 50% of the heat from a few hours of operation of one machine per week if the heat exchange input fan happens to be functioning while you are drying clothes. If you forget the filters, your whole system could become easily clogged up with lint. Although a mathematical case can be made for such an installation -- I think it's a false economy and a lot of bother. I simply vent my dryer outdoors, summer and winter.