TrueCrypt is an extremely popular and easy-to-use encryption service. TrueCrypt encrypts entire disks and volumes which make it very useful for encrypting large amounts of data at one time, and it can even encrypt your Windows drive. However, it does not encrypt single files at a time which can be useful to may companies that would rather encrypt files separately or in real-time.
BoxCryptor encrypts files using 256-bit AES encryption and encrypts single files and folders as soon as they are created and saved. This ensures that no unencrypted files are synced to your cloud service for any amount of time since it is encrypted in real-time.
These are both good options depending on what your encryption needs or wants are, and both can be useful in preventing unauthorized access to important client or company data. Both programs work well with all of the popular cloud services. TrueCrypt is free, while BoxCryptor is free for non-commercial use on 1 drive, with prices ranging from $45-100 for upgrades.