Edit: Brain Fart — I seem to have omitted about a paragraph of pertinent information.
I’ll assume you all know what Dropbox is. If you don’t, go get Dropbox now. It’s free and it’s awesome.
The only downside of Dropbox is that if you want more than the 2GB of storage you get for free, it gets more expensive, and the upper limit on how much you can store is (for practical purposes) quite low. Personally, I haven’t found either of these an issue — but thanks to my link on opensourcemac.com, I have a pretty huge “free” account. But it would be awesome to have something like Dropbox that was so big I could just stop managing what I keep on it altogether (of course, this is the problem with stuff that’s free — you waste it).
Box.com has been around about as long as Dropbox (heck, it has an even better domain name, right?) but has been targeted at the enterprise.
hubiC.com (their capitalization) I just found out about via Hacker News. It offers more free storage than Dropbox, but not quite as much as Box, and vastly cheaper paid plans, including about $140/year for 10TB. (I’m not sure how you can actually get 10TB into it, short of using a ZFS or Drobo style multi-disk volume.)
2GB vs 50GB vs 25GB
This is how much storage you get for free.
$100/year for 100GB vs. $10/month for 100GB vs. $13.60/year for 100GB (or $136/month for 10TB)
Edit: I’ve corrected the costs for HubiC.
This is how much it costs for more storage. Box gives you — by far — the most free storage but gets more expensive than Dropbox (while offering various enterprisey features). HubiC is insanely cheaper than both of them. By way of comparison, iCloud costs $20/year for 20GB, so in terms of dollars per unit storage, only HubiC is a better deal. In terms of useful features out of the box, Dropbox support is built into far more programs while iCloud offers useful functionality (notably over-the-air backups of devices and integration with Apple products) to Mac and iOS users that no other platform can (currently) match.
For Android users, the iCloud equivalent is Google Drive, which gives you 15GB free, and costs $60/year for 100GB, making it a bit cheaper (and less useful) than Dropbox.
Mac OS X Integration
All three programs appear as folders in your home directory by default, and stick shortcuts to themselves in Finder’s sidebar. Having installed HubiC and then Box after installing Dropbox, Box was very flaky when first installed. Its installer provided no feedback, and the first few times I tried to launch the application nothing seemed to happen, followed by weird broken delayed launches. Once I’d patiently killed a bunch of Box.app instances and started over it worked well.
Box and Dropbox have similar levels of Finder integration — they indicate the state of files and provide context menu items for sharing links. HubiC appears not to do anything like this, unfortunately.
All three applications provide status menus — those icons that appear in the menubar to the left of the Spotlight magnifying glass. I should note that HubiC’s icon looks like a shapeless blue blob — a blue cloud? — which is an anti-feature. The status menus of all three seem to be perfectly fine and offer decent functionality. Oddly enough, Box and Dropbox no longer keep you apprised of your usage level whereas HubiC does.
Box has one glaring defect — it won’t sync Mac OS X “bundles” (i.e. directories masquerading as files). I have no idea why — they’re just directories. It tells you to zip them up first (gee, how about doing it yourself?)
All three services offer support for all the usual platforms — although I can’t comment on how good any of them are (except the Dropbox client for iOS is decent, and all three work decently in a web browser, although HubiC’s in-browser account management is awful). I cannot yet comment on the security of Box or HubiC. Dropbox offers, and I use, two-factor authentication, and I’m pretty sure HubiC does not (but its website is pretty hard to navigate so maybe it’s there somewhere).
Conclusion
If you just want some free storage and don’t mind not being able to sync bundles then Box is a better deal than Dropbox and it’s probably quite robust given the money behind it. If you’re already using Dropbox and don’t need more storage, Box does not work as well so unless you want its enterprisey features (and you know who you are) you might as well stick with Dropbox. I can’t really comment on HubiC until I’ve exercised it by, saying syncing a buttload of RAW files to it (if I’m going to get more cloud storage, I want enough of it to not need more than one service). If you’re interested, HubiC is a damn good deal for free and it works side-by-side with the others. If it turns out to deliver the goods, I may well end up buying a 10TB plan and switching to it from Crashplan.