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!
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.]