XtraUpload v2 Beta 4 is a turn-key PHP application for setting up a file hosting enviroment. It automates the uploading and downloading of files shared by your users and is extremely customizable. It is built on the CodeIgniter PHP Framework and is compatable with a large majority of web server configurations. XtraUpload has support for multiple user groups, premium users with unlimited tiers, multiple file upload, password protected file links, file folders, image galleries, automatic image processing, loads of configuration options, skins, plugins, and so much more. Don't waste your time and money dealing with a compeditors product, XtraUpload is FREE(Modified Apache 2.0 License, open source) and very stable. The latest beta of v2 was downloaded 80k+ times in 3 months, see what everyone else is talking about!
Minimum System Requirements

* Web Server Connected to the internet(pretty standard, check out Linode.com for some great and cheap VPS servers)
* PHP v5.2.x
* MySQL v4.1+ (MySQL 5.0.x recomended)
* 12mb of free disk space (plus space to store uploaded files)
* Apache 1.3+ (Apache v2.2.x or Nginx 0.7.x recomended)

Optional Feature Requirements

* mod_rewrite support for your webserver(apache config file included)
* GD-PHP v2+
* Freetype v2
* JPEG, PNG support in GD