Charles Teague from the MS Live Writer Team just emailed me to let me know the Live Gallery is now, well, live, for Writer Plugins.  This is a Microsoft sponsored site that allows developers to upload their Writer Plugins into a central repository, where users can search them by tags, rating, category, etc..  There is also a verification process involved, where Microsoft ensures that the plugin is safe and stable.

Another great site to find Writer plugins is WLW Plugins.