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All these extensions should regard the CMSimple_XH quality standards.
(in no particular order)
Templates for the CMSimple 4.x system (which is a fork of CMSimple_XH 1.5.3) seem to be also compatible with CMSimple_XH. However they carry a back link to the CMSimple 4.x Website, which you may want to change, when using CMSimple_XH as CMS.
This list is not exhaustive. cmsimplewiki.com gave a greater collection, however including old and not up to date plugins. If you are looking for a plugin, you might also want to check the websites of those who continue to develop plugins, namely:
Incomplete list of plugins: