So, this runs the risk of sounding like an advertisement, but I'm ok with that. If a product makes you excited, it's worth saying so, and that excitement is what the advertisers are trying to sound like.
If you use multiple computers, then you should consider taking a look at dropbox. Here's how it works - you sign up with dropbox and install a (very small, pretty innocuous) bit of software and it creates a folder on your machine called "My Dropbox". Anything you save in that folder is also saved online at dropbox, and can be accessed through their website.
That's all well and good for automated backups, but where it gets useful is that if you install the software on a second machine, it will create the folder and copy the contents of the original. That is to say, every machine that has this folder has the SAME information - change it in one place and it changes everywhere.  The second and subsequent folders are all on your machine so you can access them offline - the connection is only used for syncing up.
What's most amazing to me is how smoothly it works. This has entirely replaced me carrying around a usb drive for backing up all my work - if I write it, it gets saved in my dropbox, and I know it's backed up. Then, when I crack open my netbook out on the road, I can get at the same files and pick up where I left off. Plus, if I need to grab something from another computer entirely, I can do so through the web site.
That would be enough for me, but there's one more feature that I have only scratched the surface of. Dropbox allows you to share files as well. At its simplest, this means you set a sub-folder public, and people can logon to dropbox to download the files you're sharing, but if you're also a dropbox member, you can add the shared folder to your existing folder, and it becomes part of the sync up. In practice that means that if you and a friend are working on a cookbook together, you can have a shared, remote, backed up folder called "Cookbook" on any machine either of you needs to use which has all the files you both need. If you've ever collaborated on a writing project before and dealt with mailing files back and forth, you can imagine what a lifesaver this is.
Now, there are a lot of other services that offer similar benefits, or tangential ones. There are file-sharing sites like senduit that let you pass around much bigger files, and sites like 37signals have more collaboration tools, and they're all well and good. Dropbox is pretty simple in terms of what it does and how it does it, and that simplicity makes it robust. You could get a fancier, more expensive tool and maybe use it, but dropbox makes itself so simple that it becomes hard not to use it, and that's the mark of a great tool.
For all this, I still just use the free service. Not that this isn't something I'd be willing to pay for, but I'm not a graphic designer or a media guy - all the files I need to back up and sync are buckets full of words, and those just don't get very big. If I ever find myself pushing the 2 gig limit, I'll probably look at their pricing plans, but I don't see that coming anytime in the near future. I admit, I feel a little bad about that, but I shouldn't. They're smart guys, and every user of the free service makes their product more valuable to the people who are paying for it - like fax machines, the value goes up as the network expands - because the ability to share files is more valuable with more users. Plus, given that price point, why not give it a try?
1 - Mechanically, it does this by virtue of the local copies looking for updates from the remote copy. As anyone whose dealt with remote backup knows, that does introduce one potential problem, where a document is saved in 2 different versions on two different machines which aren't connected. The fear scenario here is that one version overwrites the other, but thankfully dropbox handles it pretty intelligently - if it detects a conflict, it will spawn an additional version of the file so you can fix the problem.
2 - Yes, that means files are stored redundantly, which may not seem incredibly futuristic-elegant, but it turns out to be incredibly practical. And compared to other things, storage is cheap.