Kayak and Hipmunk get a lot of praise for their UI but if you want to use a hacker interface for finding flights, http://matrix.itasoftware.com/ is the way to go.
It's not as pretty, but is incredibly feature-rich, not limited to the US and regularly finds me prices that are significantly cheaper than anyone else.
It's not as pretty, but is incredibly feature-rich, not limited to the US and regularly finds me prices that are significantly cheaper than anyone else.