There is one special branch within HSLayers, which is not free and Open Source, but might be interesting. It contains vector editing functionalities, based on OpenLayers and UMN MapServer. It is not WFS-T, but proprietary.
Idea is: Identify one single object within the MapServer layer, it will be downloaded and displayed as vector object within the map. You can edit it or add new objects to it, after geometry is done, you can add/change attributes and afterwards, you can store everything to the server, so edit only one object at once.
There are also snapping settings, which enables the user to snap on server-deployed layers, displayed in the map as normal raster images.
For example see http://dev.bnhelp.cz/wwwlibs/hslayers/branches/editace/examples/EditPanel.html