I currently use Git to backup my things in DropBox but I haven't found this easy enough. I am not going to create cron hacks. Does there exist some built-in way for DropBox to do backups?
Dropbox saves a history of all deleted and earlier versions of files for 30 days for all Dropbox accounts. If you purchase the Packrat add-on for your account, Dropbox will save your files for as long as you have the feature, so you'll never lose a folder or file again.
See Dropbox help : How do I recover old versions of files?
Otherwise, the simplest form of backup is to, besides Dropbox, register also to other services such as SkyDrive and Google Drive and specify the same folder to be shared between all of them (or make one a sub-folder of the other).
That way you will have several copies of your files on several servers at exactly zero cost.
For a discussion of this method see
A "builtin" way to locally backup Dropbox contents is to use it on more than one machine...
If you want to have reliable, local copies of the data in your Dropbox, use a system that follows a backup regime, and have it sync your Dropbox. For the example of a Linux server which is backed up to tape regularly:
bonus points: pimp that by using rdiff-backup to create a mirror of the directory under the control of
even more bonus points: use rdiff-backup-fs to show the history in a convenient abstraction
Install one of the many services similar to DropBox (Google Drive client comes to mind).
1a. As an alternative to (1), you could try setting the dropbox folder an the google drive folders to the same folder. This might work perfectly but it also sounds like it might be a tad more unpredictable.
Regarding the initial feature request, dropbox is NOT for creating a second copy of your stuff on your computer. The whole point of DropBox is to put the second copy of your data on their servers (the data is versioned going back a number of days, which is kind of cool). In fact, if dropbox DID let you make a second copy on your computer, it would undercut the entire business model of dropbox where you NEED to access their service in order to get your data if it is ever lost.
If you must have the second copy on your computer, there are many solutions, I recommend SyncBackPro for its staggering list of features and ability to automate everything.