I use Dropbox. I have 2.75 Gb of free space, and I've got some stuff stored there, but nothing majorly there bar some documents and that. BUT:

I am running out of space on my hard drive on the Mac. Me and Mrs sheeldz both use the Mac to store photos and music. I was thinking of putting a bunch of stuff on a cloud system...

Now, iTunes in the Cloud sounds excellent. 25'000 music files is enough to take both our music collections and put it there, and then that'll free up 100Gb of space. OR I back up my photos to Dropbox, and pay $100 a year for 50Gb and that'll take all my photos off there. This will only free up a small amount. iCloud does more, as does a few others, but I was just generally wanting to know who uses what, if any.

There's also the option of setting up my own server and HDD space... one that I have thought about this...