What's a good way to wipe a hard drive for resale? I'm selling my MacBook and would like to wipe the hard drive in such a way that the current data can't be easily recovered after I reinstall OS X.

Can you recommend a free tool to help me do this?

Maybe I should mention it's an Air model, so something that's safe to use on an SSD.
From the Apple Support pages Securely erasing a disk

To securely erase a disk or partition:
In Disk Utility, select the disk or partition to erase, and then click Erase.

Specify a format, and enter a name for the disk.

Click Security Options and choose to write over the data once, 7 times, or 35 times. Click OK.

Click Erase.


Writing data over the data 7 times meets the U.S. Department of Defense (DoD) 5220-22-M standard for securely erasing magnetic media.