BoxCryptor is a free (for up to 2 GB of data) program that will let you encrypt your files using the AES-256 standard, no matter which cloud storage provider you use.
Each file is encrypted individually in real-time and stored in a folder of your choice, e.g. your Dropbox folder.
Features: You can run a program as administrator from the BoxCryptor drive * The BoxCryptor drive can be indexed by Windows search * You can disable filename encryption in the advanced mode when you create a new encrypted folder. If you do not need the additional security of filename encryption this can be useful e.g. when making use of the versioning support of Dropbox. You can also change filename encryption later by using a small command line tool.
Note that BoxCryptor does not yet support Windows File sharing.
Be sure to choose a password which you will remember or store it securely in e.g. a password manager, because BoxCryptor won’t let you recover it.