September 2017
Some of the "item" DLC were (may still be) offered for free, but none of the "story" DLC.
So not TSP, RtO, DAA, LS, GoA, WH, or The DLC Which Must Not Be Named. But those are all available with the Ultimate or GotY Editions which can frequently be found on sale for much less than if you bought all the DLC individually.
September 2017
While you can't get them for free, they're included in the ultimate edition. The version on GOG is the most complete since it includes some goodies that used to be exclusives.