Chrome/Firefox/Opera/Safari: Hover Zoom, the tool that previews full-size images when you hover your mouse on a thumbnail, has recently been embroiled in controversy over issues of malware and privacy. Imagus is a worthy replacement that works on several browsers and across most popular websites.
Zoomable objects are marked with a customisable outline. Every aspect of how the zoomed image will look can be tweaked to your liking. And most importantly, when you don't want it, you can disable the function for a single image or the whole session with customisable hotkeys.
Imagus also includes a lot of other customisations for images in your browser. It replaces the default image viewer and you can set the size that an image will be displayed (such as fit to width), zoom with the mouse wheel, and change the page background colours.