Truecrypt did pass a security audit, but since then vulnerabilities have been found. VeraCrypt, which also open source, has taken on the TrueCrypt code and improved it, including in terms of security. VeraCrypt is backwards compatible with TrueCrypt containers. You should use VeraCrypt from now on, and convert your containers to the VeraCrypt format.

Free, but...
When one mean's "free" as in:

• "Free at point of downloading": Yes

• ["Libre software definition"][1]: No
  Be advised that according to the Fedora Project's "[Forbidden Items][2]" page:

The TrueCrypt software is under a poor license, which is not only non-free, but has the potential to be actively dangerous to end users or distributors who agree to it, opening them to possible legal action even if they abide by all of the licensing terms, depending on the intent of the upstream copyright holder.

This is indeed true as it states in the [licence][lic] (Section VI.2):

YOU MAY NOT USE, MODIFY, COPY, CREATE DERIVATIVE WORKS OF, (RE)DISTRIBUTE, OR SUBLICENSE THIS PRODUCT, OR PORTION(S) THEREOF, EXCEPT AS EXPRESSLY PROVIDED IN THIS LICENSE (EVEN IF APPLICABLE LAW GIVES YOU MORE RIGHTS). ANY ATTEMPT (EVEN IF PERMITTED BY APPLICABLE LAW) OTHERWISE TO USE, MODIFY, COPY, CREATE DERIVATIVE WORKS OF, (RE)DISTRIBUTE, OR SUBLICENSE THIS PRODUCT, OR PORTION(S) THEREOF, AUTOMATICALLY AND IMMEDIATELY TERMINATES YOUR RIGHTS UNDER THIS LICENSE AND CAN CONSTITUTE COPYRIGHT INFRINGEMENT (WHICH MAY BE PROSECUTED).

A more detailed breakdown summation of the licence can be found here: [FreeDesktop.org - TrueCrypt licensing concern.][3]

[1]: https://en.wikipedia.org/wiki/Free_software "Wikipedia: "Free software""
[2]: https://fedoraproject.org/wiki/ForbiddenItems#TrueCrypt
[3]: http://lists.freedesktop.org/archives/distributions/2008-October/000276.html
[lic]: http://www.truecrypt.org/legal/license "TrueCrypt License Version 3.0"

[No.][non-cc]

TrueCrypt has not been approved under the Open Source Initiative. This is due to not following The Open Source Definition for software.

It is also using a custom licence which has been vetted and mandated as non open-source.

• Fedora: Forbidden Items - TrueCrypt

The TrueCrypt software is under a poor license, which is not only non-free, but has the potential to be actively dangerous to end users or distributors who agree to it, opening them to possible legal action even if they abide by all of the licensing terms, depending on the intent of the upstream copyright holder.

This is why TrueCrypt does not have the status: "Open Source".
(N.B. Wikipedia's status on TrueCrypt is just "source-available")

[non-cc]: https://en.wikipedia.org/wiki/TrueCrypt#Licensing_and_Open_Source_status "Wikipedia: "TrueCrypt - Licensing and Open Source status""

Truecrypt is depracated. Veracrypt has inherited the code, added new functionality and improved security. Don't use Truecrypt, use Veracrypt; it's still open source, free and every bit as useful.

You can actually mount Truecrypt volumes in Linux without installing it, but with help of cryptsetup. Mounting an NTFS partition would go something like this:

sudo cryptsetup open --type tcrypt /dev/sdc5 cryptmountpoint
sudo mount -t "ntfs" -o "uhelper=udisks2,nodev,nosuid,uid=1001,gid=1001,dmask=0077,fmask=0177" "/dev/mapper/cryptmountpoint" "/media/actualmountpoint"

Then when you're done with it:

sudo umount /media/actualmountpoint
sudo cryptsetup close cryptmountpoint

Pretty easy, if you know how. ;)
Of course, you need to replace the devices and mount points with your own values.

As of May 2014 TrueCrypt is no longer supported or maintained. You can read all about it on Wikipedia:
http://en.wikipedia.org/wiki/TrueCrypt

The official website recommends switching to use existing encryption methods built-in with most modern operating systems. In view of this the final release will only allow you to decrypt your old TrueCrypt volumes in order to migrate to other encryption tools.

However there is a large number in the open source community with an interest in continuing the tools development under a new name and license in the near future. In the mean time the most recent full-featured version (7.1a) has been made available on various 3rd party sites and GitHub: https://www.grc.com/misc/truecrypt/truecrypt.htm https://github.com/AuditProject/truecrypt-verified-mirror

Never had any problems with the full disk encryption, because of it's on-the-fly method. And I haven't noticed any performance reduction when playing games etc.
It's simply the best free software for full disk encryption. Give it a try if you have lots of files that need protection on your computer.

This is the best encryption software I've ever seen, out of a dozen of encryption software I've tried. Finding this application was a real stroke of luck (Thanks to alternativeTo). With a novice-friendly interface and explanatory wizard it provides a great way to encrypt existing partitions/drives and to create new encrypted file containers. A TrueCrypt file container can function as an ordinary volume by mounting it as a virtual disk and the concept of hidden volumes takes data security to a whole new level. On-the-fly encryption provides access to the data while keeping it encrypted at the physical level. If there were a rating system here, I would give it 5 out of 5.

The only crypto ill ever trust it just works. yes admin is an issue but don't blame the devs balm windblows for requiring admin privileges to mount a virtual drive. all apps that use low lever hard disk emulation etc require admin privileges.

The other problems sound more like windows issues than anything. Running on linux, works extremely well, absolutely no complaints here.

Bugs make it too risky for business use, spent close to 30 hours on it & concluded it's good for creating BSODs, on XP 32 at least.

Great program, but on XP you have to keep it running, because if you close it, it wont let u start it again until you reboot. Also like wpuacnt said, you have to have administrator rights on the computer u use it on, so you wont be able to use it on your computer at Work even if you have this on a portable usb drive.

See below for more info.

Truecrypt is Secure - It Has Passed the Final Audit

The disk encryption software Truecrypt, a popular tool that was endorsed by Edward Snowden, has passed a full source code security audit. This comes on the heels of the developers of the app mysteriously quitting the project and recommending that people use a questionable Microsoft solution.

The initial audit of the bootloader was performed by iSec who found only minor issues with the software that would not compromise security. The full report of the phase I audit is here:
https://opencryptoaudit.org/reports/iSec_Final_Open_Crypto_Audit_Project_TrueCrypt_Security_Assessment.pdf

The phase II audit was completed by NCC. The full report can be read here:
https://opencryptoaudit.org/reports/TrueCrypt_Phase_II_NCC_OCAP_final.pdf