Nelleke is the female Dutch version of the Latin girl's name Cornelia, meaning "horn; war horn." It is a pet form just like Nell. Pronounced NELL-eh-keh, this name also shares a fun resemblance to the name Nellary which was used by Frank L. Baum in the Wizard of Oz book "The Lost Princess of Oz," in which Princess Ozma cannot be found.
Nelleke is so rare to English speakers that I cannot say for sure it has ever been used in the U.S. There are not any very well known namesakes, but a quick Google search reveals that the name is in steady use. Nelleke Noordervliet, for example, is a Dutch writer, and Nelleke Penninx is an Olympic medalist (rowing) from the Netherlands.
Cornelia, a familiar but unused name, was once very popular. In last ranked in 1965 but had been in the top 200 when the SSA started keeping name records. In English she became more widely used in the 17th century, possibly thanks to the Dutch. We get the word cornucopia from the same root as her name.
There was also a high society Roman family with the gens Cornelius, of which the well-loved Cornelia Africana was born. She was considered at the time to be the perfect Roman woman and matriarch. Another Roman Cornelia was Cornelia Cinna (minor), a wife of Julius Caesar. When Caesar was confronted with the demand for divorce, he refused and chose to be proscribed and without her inheritance instead. They stayed happily married.
Several other well known namesakes emerged throughout history - one of Vanderbilt fame, one a suffragette, and one an aviatrix during World War II. Most recently, Cornelia Funke is the author behind the young adult Inkworld trilogy of novels, and there was a movie made not too long ago for this series. Cornelia is also a literary name, having been used in Anne of Green Gables.
Connie, Celia, Cora or Corie and Nell, Neely or Nellie could all make nice nicknames. She still ranks on popularity charts in the Netherlands and Sweden. In 2013 Cornelia was given to 36 girls, the most it has been used since the 1990's, but it hadn't been used more than 100 times in a year since the late 1960's.
Kornoelje, from the same root word, is the dogwood plant in Dutch (botanical name Cornus). It can be found as a surname.