(relatively) Cheap and cheerful multi year package on a web host.
Space larger than most "cloud" services with ultra fine grain access should I deem it necessary via any number of methods rather than nasty web frontend and stupid standalone application.
Runs one of my email setups
Can stick anything with php, CGI and mysql and have it run and more (python, perl, other databases and such) if I bother to set up the machine to do more.
Can SSH into a machine and do something remotely not to mention use it as my frontend for something.
This means I have a photo gallery if I am too lazy to cook up a web page with it on, a music/video player assuming I need it, storage for any random nonsense rendered as a web page, any number of content management options and more.
If I had to actually do something properly though probably something from amazon as they seem to pretty much get it where the others (I have not tried Ubuntu One) are more interested in selling me a branded service for things I have been able to do for years with Microsoft's one probably only being considered by virtue of it kind of being able to be tied to exchange and that is more for clients that have lumped themselves with exchange.
This, if you're already running on a web host. Rsync and a web server, and you're set.
If you're not running a web server, you're probably looking for a cloud storage program with a fairly intuitive interface.
Here's a super helpful comparison of storage services from the Verge. Personally, I use Dropbox for academic purposes and collaboration--Explorer shell integration makes it easy to copy public links and share folders and whatnot. Plus you get more storage expansion over time despite the fact that you start out at 2GB. I've done Dropquest twice, registered with a .edu email, and invited a couple people and I now have 15GB, which is more than what I need for small-scale projects.