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 (normally with a .xpi extension), then going to the menu Tools -> Add-ons, then clicking on the cogwheel on the right, choosing Install Add-on from File and selecting 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.
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.
This plugin adds the functionality to perform an OCR (optical character recognition) for the PDFs selected in Zotero. It can add a new PDF including the recognized text, a note with the recognized text only, and HTML (HOCR) files.