For a few days now, I'm using Dropbox. It's a service that lets you store your files online, but with a really great integration into your operating system (they support Windows, Linux, and MacOS!), so that you can access them nearly everywhere very comfortably.

I'm using it so that I can now easily access some shared files on my netbook, my laptop, my office laptop, and even my iPhone without having multiple copies floating around! Before that, I always had to copy a document on my thumbdrive and carry it with me (and no, I did not always have the latest version with me, and yes, I sometimes had a few copies of a document....)

The 2GB version is free (which can be slightly expanded, please see the end of this posting for more info), if you need more space they offer a 50GB and a 100GB version for a monthly fee.

Their Features page gives a good overview of their service:

File Sync
Dropbox allows you to sync your files online and across your computers automatically.

File Sharing
Sharing files is simple and can be done with only a few clicks.

Online Backup
Dropbox backs up your files online without you having to think about it. 

Web Access
A copy of your files are stored on Dropbox's secure servers. This lets you access them from any computer or mobile device.

Security & Privacy
Dropbox takes the security and privacy of your files very seriously.

Mobile Device Access
Free Dropbox iPhone app

Also, if you edit or delete a file, a history of the change is kept and you can go back to a previous state within 30 days:

You can also see what was changed when in a nice overview tab (web version):

Files can be stored within folders (web version):

Here's the menu for a file as seen on the web version:

As I said, Dropbox can also be tightly integrated into your operating system. As I'm using Windows, I'll show some examples of the Dropbox Window 'client' here.

During installation, you can specify where Dropbox should store the documents. Yes, that means a local version of the documents inside your Dropbox will be available!
After the installation, you can find the Dropbox symbol in the taskbar:

A single click opens the settings for it:

Double-clicking opens the location that you specified during the installation (of course you can also simply navigate there through Explorer):

Additionally, for the folders within your Dropbox, you get the following menu for viewing deleted files and sharing the folder when you right-click:

This is how the contents of the folder look like. Not much difference to a regular folder in Windows, but the difference is that any changes made here (e.g. copy&pasting a document into here, or editing a doc) will be synchronised to the Dropbox server, and from there then to all other locations that you set up. For example, if I upload a document via the web interface from my office laptop, I can later on access that document on my laptop, netbook, AND iPhone.

The difference between the Windows version and the iPhone version is that the documents in the iPhone version do not get synchronised immediately, but only when you access them. But you have the option to set favourites, which are documents that get downloaded immediately and are available for 'offline access'

Startscreen for the iPhone application

Overview of all folders inside my Dropbox:

Inside the 'SharePoint' folder:

Viewing the document list:

My favourites (documents that are stored directly on the iPhone):

Here's how an Excel spreadsheet looks like on the iPhone (in landscape mode):

And this is the internal PDF viewer (portrait mode)

 

As I said before, the service is free for the 2GB version. Additionally, if you refer a friend or sign up through a referral, you get an additional 250MB:

So if you're interested in testing or even using it, here's my referral link: https://www.dropbox.com/referrals/NTI4MzQ1ODQ5

PS: If you complete the first 5 steps on the Getting Started page, you'll get an additional 250MB: