(http://i55.tinypic.com/3482bzp.jpg) (http://i56.tinypic.com/1heb8h.jpg) (http://i56.tinypic.com/167krwy.jpg) (http://i53.tinypic.com/fl9hxl.jpg)
On Air (http://www.appbrain.com/app/on-air-(wifi-disk)/com.bw.onair) is a free app for Google Android which lets you transfer files to and from your device over WiFi. In order to pair the app with a Mac you can use the AppleTalk protocol. Windows and Linux computers can connect to the phone using WebDAV or FTP. The developer recommends using BitKinex for WebDAV support.
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Basically, all you have to do on your phone is fire up the app, hit the button that shows Windows, Linux, or Mac icons to choose your protocol, and then hit the big power button in the middle of the screen to activate the On Air client. It will show you the URL to enter into your FTP client or other software in order to pull up a list of files and folders on your SD car, as well as a password.
On Air is available as a free download from the Android Market (http://www.appbrain.com/app/on-air-(wifi-disk)/com.bw.onair).