In a clear play to get Dropbox users over into their own cloud storage camp, Microsoft is offering an extra 100GB of OneDrive storage for free for a year. All you need to do is sign into OneDrive (or create an account) and let OneDrive send a file to your Dropbox account to verify you are a Dropbox user. Then you should have an additional 100GB of space for 12 months.
Yes, it's only a year, but it's still free storage space and there's no reason you can't use OneDrive and Dropbox (and other cloud storage services) in tandem if you want.
Here's the promo link: