Check out CatMDEdit. It's open-source and I think its pretty useful for editing metadata related to spatial data. From their website:
CatMDEdit is a metadata editor tool that facilitates the documentation
of resources, with special focus on the description of geographic
information resources.
It's also ISO compliant:
Metadata edition in conformance with "ISO 19115. Geographic
Information - Metadata" standard (ISO 19115:2003/Cor 1 2006, ISO/TS
19139:2007 XML encoding). Edition interfaces adapted to different
metadata profiles:
ISO 19115 comprehensive metadata model.
ISO 19115 Core metadata for geographic datasets.