Zotero is open-source, and it is easy for anybody to create additional tools to add functions to the program. Over the years, many add-ons have been created, and give the users options to personalise their Zotero according to their needs and preferences.
To install these add-ons, the usual procedure consists of downloading the file on your computer, then go to the menu Tools -> Add-ons, then click on the cogwheel on the right, choose Select Add-on from File and select the file.
A complete list of the Zotero add-ons can be found here.
ZoteroPreview adds a Preview tab with a citation preview for the item. It uses your default style (you can change it in the Preferences menu, Export)
Another option to preview a reference using all the styles you have in your Zotero: the style preview pane, a built-in feature of Zotero. Select Preferences in the Edit menu -> Cite, and click on the Style Preview button. Select a reference and click on Refresh.
Zutilo is an extension that adds several functions to Zotero, such as:
Go to the menu Tools -> Zutilo Preferences to choose the functions you wish to use. Each time you will right-click on a reference, you will have an additional Zutilo menu with these functions.
Zotfile is a Zotero plugin to manage your PDF attachments: automatically rename, move, and attach PDFs (or other files) to Zotero items, sync PDFs from your Zotero library to your (mobile) PDF reader (Ipad, Android tablet...). After highlighting and annotating PDFs on your laptop or tablet, Zotfile can automatically extract the highlighted text and the annotations and save them in a Zotero note.