Here is a lighting blog post about some useful Plone add-ons.
silvuple: Force admin user interface into your native language to make it easier to manage foreign sites. List and manage LinguaPlone translations site-wide.
imageportlet: Create banners and carousels easily using portlets. Good combo with Products.ContentWellPortlets.
visualtitle: Have a different page and navigation title. Allows you to use a shorter content title for navigation tree.
collective.scriptedredirect: Use throught-the-web Python script to create redirect rules, replacing Apache and Nginx redirects with easier and more powerful alternative