jfMagnify is a jQuery plugin that creates a magnify glass effect. This plugin will magnify html content, not just images.
MagnifierRentgen is a simple, lightweight, responsive magnifier hover plugin. (magnifying glass effect)
SergeLand Image Zoomer is free jQuery image zoom plugin the magnifying glass effect.
mlens is a jQuery plugin that simplifies creating this magnifying glass functionality so much.
It has some parameters like the shape of the lens (circle or square), lens size and the options for customizing the border.