TeamCity has a plethora of plugins that can be used to extend and simplify the way TeamCity works. Many of the functionalities/features of TeamCity that come out of the box are in fact plugins that are bundled with TeamCity. These bundled plugins begin their life as external plugins that have to be installed, and then became bundled due to their usage and usefulness.
Most of the plugins available for TeamCity are listed at http://confluence.jetbrains.com/display/TW/TeamCity+Plugins.
The list of bundled plugins in a TeamCity server can be seen from Administration | Plugins List as shown in the following screenshot: