Unquestionably, the top volume generating mortgage lead providers are: Nextag, LowerMyBills, and LendingTree. Here is a current measurement of traffic to each of their primary brand websites:

Interesting trends include an aggressive surge by Nextag:

Precipitous drop by LendingTree:

And a notable switch in leadership between LendingTree and LowerMyBills:

LendingTree announced today, to customers and lenders, that several former employees were suspected of sharing passwords with unauthorized lenders. This LendingTree security breach may have given these unauthorized lenders access to customer qualification forms.
LendingTree addressed their customers in a letter, with this information:
Recently, LendingTree learned that several former employees may have helped a handful of [...]