Hidden-City Ticketing.
http://matrix.itasoftware.com/Using
Searching a one-way from LGA-CLE on 07/06 comes up $500 for a nonstop.
To find a hidden-city ticket:
Search for a one-way from LGA::CLE and then going to CLE and cities within 300 miles and it will come up with the same non-stop flight (along with a throw-away leg to PIT) for $140.
Just don't do it too often and don't check luggage!
There's lots of other useful codes if people want similar demonstrations.