TrueCrypt is the best, free, open-source, cross-platform, on-the-fly encryption software, period.
Version 5.0a adds Mac OSX support, entire disk partition support, including the System partition, performance improvements and more!
If you have sensible information, TrueCrypt is a must-have.
TrueCrypt Web site : www.truecrypt.org
grc.com TrueCrypt review by Steeve Gibson : http://www.grc.com/SecurityNow.htm#41