Apple may go in this direction one day, but for now their MobileMe cloud service is moderately useful for e-mail and address book, less useful for calendar (too many compatability issues), and utterly useless for data - their iDisk seems OK for backup and nothing else.
Dropbox and SugarSync seem to be the two big programs out there. I’ve got them both on my iPhone, my Mac, and my Windows work computer, although I’m still figuring out how to use them; I’m pretty sure they’re useful across multiple platforms, i.e. Android… Both are free for low data amounts - 2GB or so? - but you can pay more for more data. Dropbox syncs everything you put in Dropbox, which effectively becomes another folder on your computer. SugarSync synchronizes any folders that you indicate, so in that sense it interferes less with however your computer is already set up. For now I’m only comfortable using those for projects, not for all my data.
Your own music files don’t easily work in the cloud yet. iPhone apps like ZumoCast make this possible, but it’s still a bit kludgy; people also use Dropbox for this. There are rumours that Apple is going to do more with cloud music, but not much has happened yet. On the one hand, it’d be great if I could access my iTunes library anywhere; on the other hand, iTunes is already a bloated monstrosity.
I suspect things are going towards cloud computing across a number of platforms: for example, Amazon is handling this very well with their e-books. Any Kindle book I buy is available to me on my iPhone, my Kindle, my Mac desktop at home, my Windows desktop at work, and through any internet browser (but, due to Amazon’s DRM, unfortunately not on any other e-reader). It will sync notes and underlining, and always take me to the most recently read page in the book.
Apple is doing a pretty good job of keeping my address book and mail synchronized; Picasa/ Google seems good for photos. If I ever had to do group projects, I’d probably use Google Docs.
This New York Times article, “10 Ways to Get the Most Out of Technology”…
… talks a fair bit about cloud computing - they recommend Picasa to store all your photos, for example. I haven’t gone that far yet, and Delicious’s uncertain fate has me skeptical about storing things in the cloud, rather than just using cloud services for backup or for “currently operational” projects.
Prompted by your question, I finally had a look at Picasa’s rates: 1GB free; 20GB for $5USD/year; 80GB for $20/year; 200GB for $50/year. My own photos folder is currently at 70GB. $20 seems OK.