If you use Linux kernel 2.6.x, you can use dm-crypt to encrypt your filesystem. On RedHat, it's installed by default, unless you did the bare minimum installation. On Ubuntu and Gentoo, you get can them from the package management system, - just search for "cryptsetup" and then install the package.

Here are some documents on how to use cryptsetup:

• Linux Encrypted Filesystem with dm-crypt
• Cryptsetup /tmp /swap & /home on LVM
• HOWTO: Automatically Unlock LUKS Encrypted Drives With A Keyfile
• 7 Steps To An Encrypted Partition (local or removable disk)
• Protect Your Stuff With Encrypted Linux Partitions
• Protect Your Stuff With Encrypted Linux Partitions (Part II)

Here are some additional information on cryptsetup and other disk encryption softwares.

• dm-crypt: a device-mapper crypto target
• LUKS project page
• Technical feature comparison of different disk encryption softwares

TrueCrypt is very popular among Windows users. But I don't have very good luck installing it on Linux. It has not been included in most package management systems. The TrueCrypt download area provides download for Ubuntu and Suse. Still I failed to start truecrypt.

Another option is to use EncFS:

• EncFS Home Page
• EncFS Home Page Extended Introduction
• EncFS Wikipedia Article
• How to get started with EncFS in three easy steps
• Ubuntu Forums HOWTO: Encrypted directory with EncFS
• HOWTO: EncFS Encrypted Filesystem in Ubuntu and Fedora GNU/Linux