Where can I park for free?

That depends on the time of year and time of day, so be sure to check the posted signs that designate the color of the lot - they will also give you the times they are restricted. 

Generally without a permit:
Yellow/Central District lots open after 4pm and on weekends.
Red/Gold/Blue open after 5pm and on weekends. 
Housing open on weekends only.

On weekdays, the closest free parking can be found on city streets near campus (be sure to obey city street parking signs as well). Always double check! That's the best way to ensure you don't receive a parking ticket.

You can check out all parking rules (check page 5) here http://parking.ku.edu/sites/parkingdev.drupal.ku.edu/files/docs/parkingrules.pdf.