VeraCrypt is a free and libre source (open source) disk encryption software brought to you by https://www.idrix.fr
Features:
• Creates a virtual encrypted disk within a file and mounts it as a real disk.
• Encrypts an entire partition or storage device such as USB flash drive or hard drive.
• Encrypts a partition or drive where Windows is installed (pre-boot authentication).
• Encryption is automatic, real-time (on-the-fly) and transparent.
• Parallelization and pipelining allow data to be read and written as fast as if the drive was not encrypted.
• Encryption can be hardware-accelerated on modern processors.
• Provides plausible deniability, in case an adversary forces you to reveal the password: Hidden volume (steganography) and hidden operating system.
• Multilingual. More than 35 languages. List at https://sourceforge.net/p/veracrypt/code/ci/master/tree/Translations
• More information about the features of VeraCrypt may be found in this documentation at https://veracrypt.fr/en/Documentation.html
VeraCrypt is based on TrueCrypt 7.1a at 
TrueCrypt . In comparison to TrueCrypt, VeraCrypt adds enhanced security to the algorithms used for system and partitions encryption, making it immune to new developments in brute-force attacks.
For example, when the system partition is encrypted, TrueCrypt uses PBKDF2-RIPEMD160 with 1000 iterations whereas in VeraCrypt we use 327670. And for standard containers and other partitions, TrueCrypt uses at most 2000 iterations but VeraCrypt uses 655340 for RIPEMD160 and 500000 iterations for SHA-2 and Whirlpool.
VeraCrypt is licensed with both Apache 2.0 and TrueCrypt 3.0 licenses. Per https://sourceforge.net/p/veracrypt/code/ci/master/tree/License.txt#l24
Comments and Reviews
Pros
Cons
Have used it on Windows and Linux systems and have worked flawlessly.
10/10 would recommend!
During the past 2 years have tried to use VeraCrypt but always I get the many times reported "VeraCrypt::Elevator::Elevate:710" error. However it seems that nobody cares.
Been using VeraCrypt for over 4 years to encrypt my drives. Works perfectly both on Windows and Linux, and it's super fast and secure.
Easy, Secure and strong encryption. I use it for years by now
VeraCrypt is the gold standard of private encryption.
It is a free, open-source disk encryption tool that has been third-party audited. You can't get much better than that. With closed-source software, you can trust the name. The bigger the name, the more they want to avoid damage to reputation. With non-audited open-source software, you have to place blind trust often when dealing with complicated encryption.
Although its for disk encryption, you can think of it like file encryption. You don't need different encryption software to "lock" folders. You could also just archive folders and encrypt with something like 7Zip.
For the cloud, there is some limitations, especially with large containers. The big cloud services like Dropbox can already update individual files rather than the entire disk, but some less well known ones can't. For that, you might consider cryptomator. But it's not completely audited by a well-known third party. Though most of the cryptographic libraries it uses have been.
But you don't need to worry about anything other than things like IP logging and metadata with Dropbox or other commercial brands. Because you are using this as client-side encryption, those services can never see the contents. Even the government can't see the contents if you have a good password.
Free, open-source, intuitive... I don't know what else to say.
Trustable, multiplatform, you can choose between multiple enryption methods at volume creation. Encryption is strengthened with user-generated random data. Compatible with older TrueCrypt volumes.