Recently, I happened to see an ad for Dropbox while I was browsing. I'd seen the logo before but I didn't really have any idea of what it was besides the obvious implication, so I decided to check it out.
Dropbox starts you off with a free 2GB, which is nice, but will give you more if you refer people (right now, it's 250MB per referral). At the moment, I'm up to 4GB and am trying to get up to the 10GB maximum for free accounts. Paid accounts allow you to get either 50GB ($9.99/month) or 100GB ($19.99/month).
In any case, as it turns out, Dropbox replicates the functionality of iDisk. If you aren't familiar with iDisk, the idea is that you have a hard drive on the web and can access it as a regular folder on your computer. Dropbox takes this a few steps further though.
As I was surprised to find out, Dropbox works for Macs, PCs and they even have an iPhone app. As far as I know, they don't have an application for Windows mobile devices yet but they've got it on their to-do list. Incidentally, I already use it on all of my computers (work and home) and got my wife to start up an account for herself. So far, it has worked flawlessly across all of the devices it's been installed on.
Everything you save to your Dropbox folder will have a 30-day history of revisions (change history), so you are able to revert back to older versions. They call it an 'undo history,' which makes sense. For developers, this is an indispensable feature.
Not only does Dropbox support public folders (like iDisk does) so you can share your files with friends/family, it also gives you the option to have a shared folder. A shared folder will show up as a folder in the Dropboxes of all parties involved, just like all of their other folders. So if you're collaborating with other people on anything within reason, Dropbox makes for an excellent solution. It's as if you're both working on the same hard drive ... with a slight delay for sync.
Like iDisk, Dropbox is an excellent solution for back-ups. Currently, I use Mozy which is about $5/month per computer you back up but they offer unlimited space. I'm considering switching to the $9.99/month 50GB package just for the simple reason that I can back up any of my files and access them from anywhere (even my iPhone).
So far, I've been using the site as my 'local' folder on a few sites I develop and it works 99% flawlessly. Once in a great while, I'll try and save while it hasn't finished syncing yet and it will give me an "Unable to upload" error. But I just wait a second, upload again and everything works perfectly.
Needless to say, I highly recommend Dropbox as either a Web Development solution or a Back-up solution (or both). Click here to try Dropbox. Oh and by the way, if you use my link, we both get an extra 250GB.