For editing and using SKOS models, try TopBraid Composer. There are various tools for defining and using SKOS - displaying broader/narrow relations in a tree view, displaying nodes by skos:prefLabel, constraints for SKOS data quality, etc.
Also see the Enterprise Vocabulary Network (EVN), which is Web-based end user product that works with SKOS models out-of-the-box, and with a couple of small setup steps will support concurrent multi-user editing with audit trails. This includes version control and provenance features necessary for collaborative ontology editing. You can design models and evaluate EVN with TBC-ME (free trial), and the general SKOS modeling tools are available in TBC-Free.