On May 31, Nebraska State Senator Laura Ebke announced that she had changed her registration from Republican to Libertarian. See this story.
She lives in Crete and was last re-elected in 2014. She is up in 2018 and has not decided if she will run again.
The last State Senator in Nebraska who was registered in a party other than Democratic or Republican was Ernie Chambers, who was a member of the New Alliance Party during 1988, and was that party’s nominee for U.S. Senate. Before that, the last third party members of the Nebraska legislature were in the Progressive Party, and were elected in 1922, 1924, and 1926. Thanks to Andy Craig for the link.