Uploadify is a jQuery plugin that allows the easy integration of a multiple (or single) file uploads on your website. It requires Flash and any backend development language. An array of options allow for full customization for advanced users, but basic implementation is so easy that even coding novices can do it.
Uploadify is available under the GPL and MIT licenses. This allows Uploadify to be used in a number of applications and commercial products.

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Documentation
Implementation
In it's simplest form, Uploadify is very easy to implement
<body>
<input type="file" name="fileInput" id="fileInput" />
<script type="text/javascript">
$(document).ready(function() {
$('#fileInput').fileUpload ({
'uploader' : 'uploader.swf',
'script' : 'upload.php',
'cancelImg' : 'cancel.png',
'auto' : true,
'folder' : '/uploads'
});
});
</script>
</body>
The plugin offers various options to control & customize the upload interface like:
- files types
- max. number of simultaneous files
- auto or manual start of uploads
- size limit
- callback on every step (onSelect, onCancel..)
- & more..
And, implementation is as simple as any other jQuery plugin; just by calling a function.