Backup Assistant for Excel Description:

The Backup Assistant is designed to help you make backup copies of your workbooks before you make changes to them. One of its most useful features is to prompt you to backup a file immediately after it is opened! Eliminate the worry about messing up a file and not having an up to date backup.

You have the option of backing up just the active file or selecting a group of open files to backup.
Files can be backed up to a default directory or to any directory of your choice. They can be saved by their names or have the date and time added to their names so that you can easily see which backup file is the latest.
Options include a quick access button, filter settings on files to be prompted to backup to exclude files, ability to turn off the prompt to backup on file open.