Mac users have FileVault and Truecrypt is a free and open source disk encryption utility. If you are the type who needs to secure your data from prying eyes or have confidential corporate data on your portable computer and/or portable storage device, I highly recommend using this. What I find interesting about Truecrypt is its on-the-fly encrypt/decrypt feature as opposed to encrypt/decrypt on volume mount (ala Filevault). This feature saves you precious HDD space by not having two versions of the file (encrypted and decrypted) but the encrypted file is decrypted when accessed/used by an application and in memory. What makes it even better is that it does not need to have the entire file decrypted in RAM before you can use it. Cool, huh?have Truecrypt.
I just wish that Truecrypt is available on Mac OS X, Solaris and FreeBSD but if you are using Linux and Windows, what are you waiting for? Secure your data… NOW! :)