Roku's History Channel app has some "unlocked" episodes, some require cable validation.

Much to my surprise, they seem to be streaming season 2 of The Vikings without validation. I streamed episode two yesterday (epi 1 is still up). Not only did it not require validation, there were no commercials. It stopped and rebuffered 4 or 5 times; I think it was segmented for commercial insertion but none were inserted.

The week before I watched on computer via History Channel website and it had commercial breaks (about 10 minutes plus 44 min. show).

Other options: Amazon is streaming season 1 free to Prime members but selling episodes for season 2. Hulu (non-plus) has free episodes with commercials.

RAGNAR! RAGNAR! RAGNAR!

Edit: Just watched episode 4 on the Roku History Channel app. The breaks described above are now filled with commercials. Not sure it is better or worse than any of the other options.