Gmail just released an update to their interface allowing the user to select more than one file for upload by using the CTRL-button. How do they do that? You can read about the new feature and see a screen shot here:
http://gmailblog.blogspot.com/2009/02/updates-to-attachments-multi-select-and.html
You will need to find flash-based sollution, like Google did with Gmail. You can try this jQuery plugin that offers exactly that: jQuery File Upload Plugin
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