Dropbox and Ubuntu One are Cloud service that provides storage service of folders and enable you to sync them between multiple machines. In old days, i was sending files to myself by emails to have the same files on different machine.
Dropbox offering free 2GB storage with the ability of sharing folders between different users, or publish certain files to the public (like publish certain images and put in a website or blog or send a link to it to a friend). Dropbox can be used on Linux, Windows, MAC and Mobile.
Ubuntu One is nearly the same but offering more free space of 5 GB, but runs only on Ubuntu 10.10 or newer, Windows and MAC OS not yet supported. So they offer more space but make sure all your machines on your cloud should be Ubuntu 10.10+.
Also one of the drawbacks of Ubuntu one is that it doesn’t mention if the data stored are encrypted or not. but as seen on forums most probably not encrypted which is a big issue for a lot of users. While Dropbox encypt data with AES-256 encyption.
So as i have 3 machines, with different distributions and versions (Ubuntu 8.04, Ubuntu 10.10, Ubuntu 9.04 and CentOS 5 + Windows Vista) and for Security (Encryption) I had to go with Dropbox. Sorry Ubuntu.
On Dropbox, Folders and files in your dropbox folder on your home directory are syncronized with all your machines linked to your cloud. Also you can browse the data on your cloud using Dropbox website
Learning ... 
Yeah, it’s a cloud era! Microsoft Azure, Apple iCloud, Ubuntu One, Dropbox …
That makes me wonder, how much should we trust these companies not to compromise our data! They’re working hard to convince you to put your entire digital life on their servers! and that’s all for free! Doesn’t that ring a bell?!
And BTW, it isn’t so hard to believe. We’ve already been hearing about Facebook selling user data to third party advertisers and government agencies!
yes, this for sure the cloud era, and you shouldn’t trust them for sure, as for example Amazon S3 cloud has removed the contents of wikileaks from their cloud data centers, so it seems that they have access to data and encryption keys for data, for example Dropbox don’t make you create your own keys for your data encryption but they have database to link the AES-256 encryption keys to your account so at anytime they can get access to your data easly the same for Amazon S3 datacenter after the wikileaks removal it has been known that it has access to all data on there center without acceptance of user. The solution for that for me is easy, just put you files on cloud encrypted, i made a small python script for that to encrypt the dropbox folder.
Or even better, create your own cloud!
http://www.maclife.com/article/howtos/how_build_your_own_online_cloud
http://www.informationweek.com/news/software/enterprise_apps/226200230