We're quickly turning into a cloud-based internet society. We want everything available online, everywhere we go. That has its benefits, for example, you don't need to carry around your laptop all the time if you want to access your files. Dropbox, you can just use your mobile phone or another computer that is handy to get to your pictures, or documents. It's very convenient and seems like the logical step in the next evolution of accessibility.
"The Cloud" buzzword is nothing more than a new term for the "Internet" so before you get to hung up about that, just remember that the cloud is just a marketing term to sell you onto these services like Apple's new storage service, or services offered by companies like Dropbox. Yes, they're good services, and they're amazing in their capabilities. I can save a picture to my desktop, and within seconds it will be available on my laptop, ipad and any other computer I want. Pretty cool if you ask me.
But maybe you don't want to have your stuff stored on somebody else's servers. That is all that "the cloud" is; somebody's servers on the internet giving you storage space and making it available easily. Recently Dropbox went through a privacy faux pas and this put a lot of people on notice. Do you really own your data once it's on the cloud? Only time will tell, my advice is to keep local backups and keep your very private information away from these services.
What if you want to have your data available across all your computers, but you don't want to trust online services to do this for you? Do you have alternatives? Of course you do; Open Source application Unison to the rescue. Yes, you could manually copy your files when you need them, but who wants to do that? I don't, and even when I try to use this method, I often forget to copy the files I actually need.
If you're familiar with Rsync, then you could call Unison, Rsync on steroids. Unison can keep data synchronized across networks, much like Rsync does, except it goes one step further to help you deal with conflicts and generally doing the right thing. It's available for all platforms, with Linux being the easiest one to use, then OS X and finally Windows being the last one. I need to let you know that this isn't for the faint of heart, it is a solid application but setting it up might require a little more than your usual click, click, agree, click installation procedure.
In OS X, you'll need to use Macports to get Unison; unless you want to build from scratch (and good luck with that). On linux, most package managers have Unison in their repos so you should be able to do something like "yum install unison" and on Windows, you'll need Cygwin to make it work.
Unison is really useful when you want to synchronize large filesystems and Dropbox or other services don't provide that capability or the cost would be prohibitive.
I use it to synchronize my entire clients directory between my iMac and my laptop, this way I can do work my big screen at home, and take it on the road when I work on-site at a client's office. Most of the files I work with are too large for Dropbox and I rather not put them on a server that I don't own or have control over. All in all, I use it to keep about 60 GB worth of data synchronized.
Once setup, unison is very handy and you can trigger it with a simple script command or you can even set it up to work automatically on a schedule. I've been using it for about a year now and it works perfectly.
If you want the benefits of sychnronizing your files across many computers, but don't want to use a public service, you should give unison a try. It might be just what you're looking for.