My premier and favorite cloud storage would be Dropbox. You can install it on Windows, Mac, Linux and most popular smartphone OS. I love the way my files sync seamlessly between all my machines and the speed that they updates. Apart from that you can also access your files stored on Dropbox from their website which happens to store old or deleted versions of your files in case you changed your mind. Dropbox gives you 2GB of free space for starters but you can gain up to 10GB more free space using their referral system. Just refer Dropbox to your friends using your private referral links and once someone started using Dropbox using that link, both you and your friend gets 250MB free storage space. I've been doing this for so long that I now have 9GB of free space with this cool referral system.
My second preferred cloud storage provider is Box.net. At first I didn't like using Box.net much because they won't let me sync files on my computer without paying for their Premier plan. Then after they gave me 50GB of free space for using the Box.net iOS app, I changed my mind and uses Box.net to store all 8GB of my music files online. That said the lack of desktop sync is still a bummer.
While Dropbox and Box.net is good for storing small to medium sized files, I usually store most of my large-sized files on Microsoft's Skydrive. First of all they give a generous 25GB of free storage space and I can upload up to 100MB file in size there. I use Skydrive to backup all of my digital photos, computer drivers, software and archived office documents. Skydrive is accessible through a browser, iPhone and Windows Phones although you have to have Silverlight installed on your desktop browser first. All you need to use Skydrive is an email address so sky's the limit.
Otixo does not provide you with any cloud storage but this nifty little website allows you to access, view, upload and download all your files from many popular different cloud storage provider from the same place. Otixo supports Dropbox, Box.net, SkyDrive, SugarSync, 4Shared and Google Docs just to name a few. Bare in mind though, Otixo has a bandwidth limit to how many files you can upload or download at a given time so you might not want to use it to store very large files.
Apart from that I also use a number of other popular cloud storage provider like SugarSync, Mediafire and 4Shared. SugarSync is similar to Dropbox or Box.net but somehow I often had problems deleting unwanted files from my account and that proved to be quite frustrating hence why I don't use it so much. Mediafire and 4Shared are two of the most popular web-based cloud storage provider out there. In fact there are countless web-based storage like those two but I don't really trust most of them to store my files or sensitive data plus for all you know they might shut down or disappear next week. Reliability, security and established names are among the top priority in choosing your personal cloud storage apart from the obvious space and features of course. Why Google Docs is not mentioned here? Well first of all they gave you a measly 1GB of space and second, they are only accessible from a browser. So until a worthy Dropbox-like app comes Google, I only use Docs mainly to store my office documents.
Storing all your important and sensitive files solely on your computer hard drives is not really advisable. Your data could be gone in a second from faulty hardware or malware attack. Therefore apart from offline external hard drives, cloud storage is the way to go for keeping all your files safe, in the cloud.
