There was talk about having a repo for all inkscape extensions but nothing so far, you can find most of them in Inkscape and some more listed here
http://wiki.inkscape.org/wiki/index.php ... RepositoryMoving zero point of the coordinating system is an often requested feature, but it's not implemented yet.
The 0.47 version has turbo snapping options, there you can select to snap to line centers, this might be what you need.