LiteZoom is a really simple magnifier script. Hover on following images to see the demos. To use LiteZoom, just include litezoom.js in your <head> and then adapt one of those examples to your need.
<script src="litezoom.js" type="text/javascript"></script>
<a href="small.jpg" rel="litezoom">
<img src="small.jpg" alt="Just a picture" />
</a>
<a href="small.jpg" rel="litezoom[5]">
<img src="small.jpg" alt="Just a picture" />
</a>
<a href="big.jpg" rel="litezoom[1]">
<img src="small.jpg" alt="Just a picture" />
</a>
<a href="big.jpg" rel="litezoom[0.5]">
<img src="small.jpg" alt="Just a picture" />
</a>