I've been a big advocate of Dropbox for a long time, but I've recently made the switch over to Copy for multiple reasons.
First, there's a lot more permanent free space. If you sign up with a referral link, you get an extra 5GB -- which gives you 20GB to start. You get another 5GB at the moment for each friend you get to signup, as well. The web interface is a bit more intuitive and useful than Dropbox's. And -- and this is key for me -- you have more control over *how* you share files, including the ability to specify read/write permissions. The finder integration is good (I'd say on par with Dropbox). On the downside, not as many apps have made use of Copy as do Dropbox (but I suspect that'll change). They're run by a good company with a strong security record (Barracuda) and I've had zero problems with the service or the apps (on my Macs and my iPhone). You can signup at
http://copy.com for the standard 15GB of space.