Windows, Mac, and Linux: BoxCryptor is an easy to use file encryption program that encrypts data on the fly on Windows using AES-256 encryption and supports encrypted folder mounting on Mac OS X and Linux. The program is ideal for Dropbox users.
We've covered quite a few ways to encrypt files in Dropbox. Open source TrueCrypt may be the most trustworthy method, but it requires you to create fixed container sizes and mount and unmount the containers each time you want to work with the files. Another option is to use encrypted zip files to secure groups of files in your Dropbox folder, but then you'd still have to unzip them.
Like previously mentioned SecretSync, BoxCryptor offers client-side encryption in a special folder where you can very simply drag files you want to encrypt and store securely on your Dropbox account. BoxCryptor encrypts on the fly and decrypts them in real-time.
The main difference between BoxCryptor and SecretSync is that folders encrypted with BoxCryptor can be mounted on Linux and Mac OS X with EncFS (a free cryptographic filesystem). Using BoxCryptor with Linux or Mac, however, requires installing a few programs and following some steps; here they are for Mac and Ubuntu.
Otherwise, encrypting and decrypting files in BoxCryptor, which could automatically be placed in your Dropbox folder, is as simple as moving files into the mapped drive for BoxCryptor. Looking in the BoxCryptor folder under Dropbox—which you should not place files in, you'll see your encrypted files.
BoxCryptor is a handy tool that doesn't really require Dropbox. You could use it as a simple file encryption program or with other cloud services as well.