Mac Lion's AirDrop feature lets you wirelessly share files between compatible Macs with no configuration or set-up required. Here's how to use it.
AirDrop, new to OS X 10.7 Lion, builds on the success of Bonjour, Apple's zero-config networking tool that's used for everything from finding printers to chatting across a local network. Although built along similar lines, AirDrop is more focused, having only one function: it wirelessly shares files between compatible Macs with no configuration or set-up required by the user.
If you're running Lion on a recent Mac, you already have everything you need to use AirDrop. The earliest compatible Macs are the MacBook and MacBook Pros from late 2008. You don't need to be in range of a wireless network or connected to the Internet. As long as you're close to another Lion-equipped Mac, you can exchange files without a password in three steps:
AirDrop doesn't need passwords and can't be switched off, but it does have a number of built-in security features. Chief among these is the fact that nobody can send a file to your Mac unless you accept the transfer. Likewise, it's a one-way operation, so no-one can use AirDrop to grab a file from your hard drive.
More importantly, if you don't have AirDrop selected in the Finder sidebar, your Mac won't appear in anyone else's AirDrop scan. If you want to avoid clicking AirDrop by accident, remove it from the sidebar either by holding Command while dragging it away, or going to Finder/Preferences/Sidebar and unchecking the AirDrop entry.
As its name suggests, AirDrop only works over Apple's AirPort hardware. You can't use it to transfer files from a wireless machine to a Mac connected by Ethernet. To do that, you'll have to revert to using shared folders.
Open System Preferences/Sharing on the wired Mac and click the 'file sharing' check box to make the machine visible on your network. Switch to another machine on your network and ensure shared resources are visible in the Finder by opening Preferences and, in the Shared section, checking the box beside 'Bonjour computers'. The wired Mac will now appear in the sidebar of any Finder window opened on that second Mac.
You can now drag files from the Wi-Fi-enabled Mac to the Ethernet-connected device, either dropping them in the latter's public drop box or, if you have a username and password for the machine, navigating its file structure to a specific location. The same works in reverse, allowing you to drag and drop files from the wired Mac to any other machine on the network.