Encryption Wizard is an easy-to-use Java-based file and folder encryption tool. It's been developed by the US military but this version is free for anyone to use.
The program is extremely small, just a 1.23MB JAR file and a couple of help files, which means it's ideal for protecting files on USB drives. And being written in Java is another advantage, as it'll work anywhere there's an up-to-date Java engine installed: PCs, Macs or Linux.
Encryption Wizard is also very easy to use. All you have to do is drag and drop the files (or folders) you'd like to encrypt onto the program, click Encrypt and enter a passphrase (or it can generate one for you, if you prefer). You then have the option to securely delete the original files, after which the program will encrypt your files using 128-bit AES with a click.
But there's plenty more functionality if you need it. If you're not happy with passphrases, say, you can use a PKI or X509 certificate, or even a suitable smart card (CAC/ PIV). There's also support for compressing files; you can add metadata to the document which won't be encrypted (handy if you've a bunch of encrypted files and want to quickly find the right one); and the program's operation can be customised in a variety of ways.