If a file is unable to go through email, mega.nz is my favorite Encrypted on disk, in browser, in transit and in the cloud. No unsecure security questions for password reset. Download and store the recovery key, there is no other way to restore an account from a lost password.
File sharing is done with links, either with a password with the link or separate.
Sync Clients for everything - Linux, Windows, Mac, browser extensions for chrome and firefox. Command line for Linux, Mac and Windows to run Mega commands from command line or scripts and all mobile ios, android, windows phone and even blackberry.
They do keep registration and payment info, but what info you give is up to you unless you pay for more storage and then they will keep your credit card info on file and it will be used if needed and any other information like your IP address(es) to find and prosecute you for anything illegal. Privacy and Terms explains what personal information they keep. They also know the email addresses when you send files to someone.
Mega cooperates with DMCA take down requests and will suspend or cancel account because of copyright and other TOS violations. If files are encrypted, how do they know? If you share a link of a movie, and everyone else shares the link of the movie, EVERYONE knows the link is a movie and Mega knows who that link it belongs to.
Sounds scary, just be careful who you share stuff with. I haven't tried it, but if you share a movie with one or two friends that don't share it with anyone else, you're probably fine. Cancel the share if you think it will go viral. Or don't share illegal stuff, spam or extortion letters from Mega. Their Transparency Report explains a lot.
For files you want to hide from everyone it works great. The only way Mega knows something is illegal is when you share it and someone complains.
Be careful what information you give Mega. Only a real email address is required if you're not buying more space than the free 50 gb, then you're giving away payment information. Only access from a VPN (including any sync clients mentioned above) so they don't have your real IP address.
I appreciate the full encryption and the Linux client and the free 50 gb. Plus encrypted Chat between Mega users. It's fast, the sync and mobile clients work well and sharing is easy and secure. It 50gb free.
Just in case. Don't fully trust anyone because anything can be exploited
MegaPWN Bookmarklet to recover your secret MEGA master key. Your MEGA master key is supposed to be a secret, but MEGA or anyone else with access to your computer can easily find it without you noticing.
This problem is illustrated by a MEGA-specific browser bookmarklet that allows the victim to break into his or her own MEGA account. A more generalized approach is outlined in Brian Kaplan's paper RAM is Key - Extracting Disk Encryption Keys From Volatile Memory. And, needless to say, if the victim installs remote monitoring software (such as a keylogger/screen grabber) on his machine, the potential security breach becomes pretty much all-encompassing.