Would you want to upload files with confidential information unencrypted to cloud storage services? If you care about security and privacy you wouldn’t. What can you do about it? A good approach would to encrypt your files using a local tool like ccrypt and then upload it to the cloud. However, you may not have your laptop with you all the time. InstantCryptor is a webapp to solve this problem.
InstantCryptor works with Dropbox and Google Drive at the time of writing. It is free of cost and claims to have 0 knowledge of your data as all files are encrypted (or decrypted) on the local device. InstantCryptor works directly from the browser, you do not need to install anything on your device.
Note that decryption fails if the file data is modified or the password is wrong. The latest implementation prepends a 32-byte checksum to make sure that the encrypted file is intact.