Are there any free econometrics textbooks available for individual use? I'm aware of Bruce Hansen's first year PhD [Econometrics][1] textbook, but I'd be interested to hear if there are any other such resources.

Notes: 
This question is similar to an existing [CV question][2], but I'm specifically asking for references that would be useful to econometricians and students of economics. 

Also, duly noted that this should be made a community wiki if appropriate (how to do that?).


  [1]: http://www.ssc.wisc.edu/~bhansen/econometrics/
  [2]: http://stats.stackexchange.com/questions/170/free-statistical-textbooks