I'm hoping to add search and replace within selection and search within files. Having a search option in the context menu would also be handy.
For searching within files I found this plugin: http://oliver.github.com/gedit-file-search/.
The ability to search and replace only selected text has been requested for years
( https://bugzilla.gnome.org/show_bug.cgi?id=150010 ) but I haven't found any plugins to do that.
A complete list of gedit plugins: http://live.gnome.org/Gedit/Plugins
Is this gedit search plugin what you're looking for? Instructions to install are on that website.
This thread is really old, but here is a little update. For gedit 3 I would recommend this plugin advanced-find over the gedit-file-search, because I was not able to install the latter with my gedit 3.4.1. Also, it works fine.