The Vikings were seafaring groups of Humans from Scandinavia, in Northern Europe.
In 1014, a band of Vikings attacked an Irish monastery in the Battle of Clontarf. (DS9: "Bar Association")
Julian Bashir ran a Viking program that Rom described as "stupid." (DS9: "It's Only a Paper Moon")
The Vikings were known to inhabit the future nation-state of Denmark, among others. (VOY: "Heroes and Demons")
In 1953, when Herbert Rossoff and Douglas Pabst almost got into a fight, Julius Eaton called for calm and remarked "we're writers, not Vikings." (DS9: "Far Beyond the Stars")
In 2372, Miles O'Brien and Julian Bashir participated in a holosuite program where they fought against the Vikings at the Battle of Clontarf. (DS9: "Bar Association")
The Viking concept of Valhalla, an afterlife for honored warriors that died in battle, is similar to the Klingon concept of Sto-vo-kor.