I really enjoy Vikings and recently started watching a new series "The Last Kingdom". It's not quite as good as Vikings, but 3 episodes in I'm hooked on the plot already. It's about a English boy that was taken by Dane Vikings and was raised by them, then returns.
Like I said, it's not as good as Vikings, but thoroughly entertaining and keeps the theme fresh while Vikings is between seasons.
Another show I tried recently was "Da Vinci's Demons". I started with very low expectations, but it's turning out to be one of the best history pieces on TV (not that it's historically accurate at all). It reminds me of The Borgias (or Borgias? Whichever one was actually good).
Anybody else try either of these series yet? Interested to see what you think.