Shipped plugins with gedit
In gedit itself
Check update: Check for latest version of gedit (Win32 and OS X only).
Document Statistics: Analyses the current document and reports the number of words, lines, characters and non-space characters in it.
External tools: Execute external commands and shell scripts. (More)
File Browser: Easily access your file system (includes remote mounts, creating new files/directories, monitor directories for changes, etc.).
Insert Date/Time: Inserts current date and time at the cursor position.
Modelines: Emacs, Kate and Vim-style modelines support for gedit.
Python console: Interactive Python console standing in the bottom panel.
Quick Highlight: Highlights every occurrences of selected text.
Quick open: Quickly open files.
Snippets: Insert often used pieces of text in a fast way. (More)
Sort: Sorts a document or selected text.
Spell: Checks the spelling of the current document.
The gedit-plugins package contains useful plugins that are (most of the time) too specific to be distributed with gedit itself. For cutting edge stuff check git repos both original as well as GitHub mirror forks
Bookmarks: Easy document navigation with bookmarks.
Bracket Completion: Add automatically a closing bracket when you insert one.
Charmap: Select characters from a charactermap.
Code Comment: Comment or uncomment blocks of code.
Color picker: Select and insert a color from a dialog (for html, css, php).
Commander: Command line interface for advanced editing.
Draw spaces: Draw Spaces and Tabs.
Join lines/ Split lines: Join or split multiple lines through Ctrl+J and Ctrl+Shift+J.
Git: Shows what has changed since last commit
Multi edit: Edit document in multiple places at once.
Session Saver: Allow to bookmark working sessions in order to get them back for further use.
Unfortunately the plugin was broken and unmaintained, so it has been removed.
Show tabbar: A very basic plugin which shows or hides the gedit tabbar.
Smart Spaces: Allow to unindent like if you were using tabs while you're using spaces.
SyncTeX: Synchronize between LaTeX and PDF with gedit and evince.
Tag List: Provides a method to easily insert commonly used tags/strings into a document without having to type them.
Terminal: A simple terminal widget accessible from the bottom panel.
Text size: Easily increase and decrease the text size.
Word completion: Word completion using the completion framework.
If you have developed a plugin which you think would be a candidate for inclusion in the shipped gedit plugins, please send an email to the gedit mailing list stating what the plugin does, where it is available and why you think it should be included.