1. I18n: Rails Internationalization
RailsCast at http://railscasts.com/episodes/138-i18n
Guides post at http://guides.rubyonrails.org/i18n.html
2. Seed-fu: Initial data loading for application (e.g. admin user, product categories)
Tutorial http://github.com/mbleigh/seed-fu A support is now built in for Rails 2.3.4
3. Thinking-sphinx: Full text search
4. Paperclip: File and image upload made hassle free
Thoughtbot wiki http://wiki.github.com/thoughtbot/paperclip
If you looking for any of the above for features in your app, I suggest you give these plugins a try. It should be good for most of the apps unless you have some really unique needs.
Write in comments if you would like to share some other useful RoR plugins.