CSW has a good presence on the server side (pycsw, GeoNetwork Opensource, deegree, ESRI Geoportal are some FOSS packages). From the client side, OWSLib is the go to library for Python folks. QGIS has MetaSearch (which uses OWSLib).
- geospatial libs like OpenLayers, Leaflet
- web frameworks like jQuery, AngularJS, and so on
- browser applications, node.js, etc.
- Unit tests (QUnit?)
- Build routines (using Grunt initially)
csw4js is still early days (thanks to Bart and others for advice), so it’s a good time to rewire things before getting deeper. Interested in helping out? Get in touch!