A simple Turtle editor, with prefix autocomplete.

If you type a prefix that is well known (e.g., rdfs:) the prefix definition will automatically appear in the preamble.

When you write a predicate starting with a prefix, you can hit Ctrl+Space and a list of properties available for this prefix will appear. If you write an object when the predicate is rdf:type or a, you can hit Ctrl+Space and a list of classes available for this prefix will appear.