All of these services have good Windows support, decent OS X support, and at least a hint of Linux support. Some of them are easier to activate using Windows than other platforms.
- Dropbox, an old standard in could storage. The referral link gets you 2.25 GB of free storage to start with. Make sure you complete at least five of the steps under the "Getting Started" tab to get another 0.25GB. Also, for a limited time, you can use the "Camera Upload" feature of the Windows beta or the Android beta to get up to 5GB more free space (so 7.5 GB total!). Then you can get 0.25 GB more for every new person you refer to the service.
- SpiderOak will give you 3 GB if you join via that link. However, during your account setup, you can then try the promo code worldbackupday to get an additional 3 GB so that you top off at 6 GB total of free storage for the life of the account. In case you already have a SpiderOak account and didn't know about the promotion code, you can still get the extra 3 GB. Just login to the SpiderOak website, click on the "add new space" (it might say "buy new space"), then click to "Change" your account, and then enter the worldbackupday promotion code. Assuming the promotion code hasn't expired, it all should work.
SpiderOak is very similar to Dropbox and yet also very different. It encrypts your files locally so that SpiderOak employees don't even have access to your data. It also does the encryption in a way that ensures that files duplicated across your machines don't take up duplicate space. Plus, you can create encrypted shares where a subset of people have access to storage but no one else. Finally, SpiderOak allows you to select multiple folders to backup on your system rather than just keeping one "dropbox" in sync. You can use it like dropbox by selecting a single folder to be synchronized across your computers, but you don't have to.
- Minus.com will give you something like 10.2 GB for clicking on that referral link. It's a relatively new service, and it has a nice on-line interface with plenty of mobile apps. It's easy to upload large files and distribute download links to people. You can mark entire folders as "Public" or "Private", and you can still share download links to private folders or even individual files inside private folders. Just be careful because the default access level is Public. You can change the default to Private in the settings on the web page.
- Memopal is a little difficult to setup if you use a non-Windows system. In that case, follow the referral link and register your e-mail. Then follow the link corresponding to your operating system and follow the instructions. I think it will say you'll be getting 3 GB for free, but then you'll actually get 3.5 GB because you registered your e-mail as a referral. One plus of Memopal is that you can guy a whopping 200 GB for US$50/year, and that rate gets discounted as much as 60% if you purchase several years at once. Unfortunately, I didn't see an easy way to configure what Memopal backs up. I didn't play with it long, but it may just try and backup every bit of media it can find (and use as much CPU resources as it wants to do so).
- SugarSync gives you around 5 GB at that referral link. For non-Windows users, you should check your e-mail after going to that link and you'll find an e-mail verification link. That e-mail verification link will bring you to the standard login and a Dropbox-like web interface. Moreover, you can find additional software support there. You get extra space for completing tasks like installing the mobile app or going through the mobile app's demo "game" that leads you through sharing files from SugarSync.
- Box.net has no referral links. However, they often offer 50 GB promotions that upgrade your free storage from 5 GB to 50 GB just for logging in in a special way. For example, back in December, I logged into my Box.net account via someone else's iPad and then logged out. That's all it took for me to get 50 GB for the life of the account.
- iDrive will give you 5 GB at that link, but it's easy to add 10 GB more onto that. Just create a fake e-mail account at Aol.com. Then add at least five contacts to the contacts list. Try adding pluses to your own e-mail address username if you want to make sure they're unique. For example, you might use your own e-mail address of firstname.lastname@example.org as well as email@example.com because most mail servers will omit everything behind the plus sign, and yet it will look like a unique e-mail address. The point is to create what look to be five unique contacts without spamming five unsuspecting individuals. Then log out of Aol.com and go back to iDrive's referral page. It will give you the ability to enter in your Aol.com username and password so it can access your contacts list. For giving it access to your entire contacts list, you get 10 GB of extra storage.
- SkyDrive is Microsoft's cloud storage offering. There are no referral links; everyone gets 25 GB of free space.
- Bitcasa is still in beta, and it won't have referral links because every user gets infinite storage. They use a dubious encryption scheme that prevents duplication across users so they can maximize on storage efficiency in their end while still keeping stuff encrypted.
By the way, if you open up multiple such accounts and want a central place to manage all of your services, check out Otixo which can connect to each of your sharing sites and create shared workspaces bridges across storage boundaries.