I'm not sure if it's still around or popular but I used Jungle Disk several years ago. The Windows client allowed you to configure your own encryption keys (fil

I'm not sure if it's still around or popular but I used Jungle Disk several years ago. The Windows client allowed you to configure your own encryption keys (files are encrypted locally before being uploaded) and S3 is the backend. It was similar to Dropbox in that you can just move files to your Jungle Disk folder and everything happens automatically and transparently.