BIBFRAME Editor (BFE) and Marc2Bibframe transformation: http://bibframe.org/tools - tools provided by the Library of Congress to describe resources using the BIBFRAME ontology and to transform existing MARC records into BIBFRAME.
BIBFRAME Scribe: http://editor.bibframe.zepheira.com/static/ - a tool created by Zephiera to describe resources using a version of the BIBFRAME ontology.
*Callimachus: http://callimachusproject.org/ - The most mature linked data editing platform that I'm aware of is Callimachus. It's unique in that it uses RDFa as a templating format, which makes it easy to create custom forms for data entry. This seems to work well when the data model is one-dimensional, but not when combining different classes in a single form (in the free version, at least).
*Graphity: http://graphityhq.com/ - Lets users define the structure of an entire app in a custom RDF "sitemap." Graphity has already started a BIBFRAME extension of its Graphity Client platform (Graphity BIBFRAME: https://github.com/Graphity/graphity-bibframe). As with Callimachus, however, the major drawback is in creating multidimensional forms that would support all the complexities of library data.
RIMMF (RDA in Many Metadata Formats): http://www.marcofquality.com/wiki/rimmf3/doku.php?id=rimmf – editor for RDA data that can import MARC21 records and export RDA/RDF.
xEAC: https://github.com/ewg118/xEAC - Although not a general-purpose RDF editor, Ethan Gruber's xEAC, an XForms-based editor for the EAC-CPF format, provides a powerful model for creating data-driven cataloging interfaces.
Cataloger's Workbench: https://github.com/pulcams/catalogers-workbench - At Princeton, they are exploring Ethan's XForms-based approach and have been working on an RDF/BIBFRAME editing app. It's less ambitious than the previous platforms, but more specifically geared toward the needs of catalogers used to working in a MARC environment. They hope to have a minimum-viable-product version of the code on GitHub very soon.
DotNetRDF https://bitbucket.org/dotnetrdf/dotnetrdf/wiki/UserGuide/Tools/rdfEditor - a Windows GUI tool designed as a Notepad replacement for editing RDF and SPARQL. It provides syntax highlighting, validation and auto-completion. A step up from hand-coding your RDF!
Fluent Editor http://www.cognitum.eu/semantics/FluentEditor/ - this is an example of an ontology editor that may have potential as an RDF editor depending on how well it can import and use the ontologies that libraries may use. Uses natural language statements to construct RDF/triples. Also a Windows-only editor, sorry Mac users!
TopBraid Composer http://www.topquadrant.com/tools/modeling-topbraid-composer-standard-edition - building, managing and testing configurations of ontologies and RDF graphs, and creating instances based on ontologies. Based on Eclipse IDE.
RDF Studio http://www.linkeddatatools.com/rdf-studio - environment for editing, browsing and visualizing RDF and ontologies
*[Both Graphity and Callimachus are compliant with the Linked Data Platform standard :http://www.w3.org/TR/ldp/. Both are open source, but also offer enterprise versions with additional features and support.]