Here's more discount travel agencies I found. I only used Travel For Less for discounted tickets for CO, NW and UA flights. The last one is PriceCostco Travel but I'm not sure whether it requires membership number or not. BTW I'm PriceCostco member.

The discounts are based on published fare; usually $25 off on fares $150-$249, $50 off on $250-$349, $75 off on $350-$449, and $100 off on $450 or more.

Except PriceCostco, others require you to call the airline and put the reservation on 24 hour hold. Then you call the agencies and they go ahead and ticket the reservation.

These discounters cover six major airlines. Usually USA (50 states) and Canadian destinations are covered but some include Caribbean, Mexican, and Central American destinations.

For further detail, check their sites.

http://www.travel-for-less.com/
http://www.traveldiscounters.com/
http://www.pricecostcotravel.com/airline.html