Despite the existence of rivals like Google Drive and Box, it's hard to argue against the fact that Dropbox is the file-syncing service. It's simple, easy, and seamless.
Dropbox's built-in sharing features fit that description, too, but if you want to password-protect each file you share, you're out of luck. That's where PassDropIt comes in.
PassDropIt is as simple as Dropbox itself. You don't need an account to use it, so it's great for one-time use. Just head to the site, sign into Dropbox (PassDropIt does not authorize through Dropbox, so it doesn't see any of your account info, as far as I can tell), and select the file you want to share.
PassDropIt will grab the file and give it a unique url and password. When you share the link, visitors will see a clean page with the PassDropIt logo and an entry field to enter the file's password.
From there, of course, your recipient can download the file. All this happens without an account. If you choose to sign up, you'll be able to manage all of your shared files from one page, complete with options to view, share, download, or delete the file from your account.
If you're pressed for time, and the file you want is still uploading to Dropbox, PassDropIt has you covered. Just copy the public link from your desktop (context menu > Dropbox > Copy Public Link) and choose the "Send it Then" option just beneath the upload button on PassDropIt. Paste the previously-chosen public url and enter a recipient's email, and PassDropIt will automatically email that file when it's finished uploading to Dropbox. Note that this file will have to be in your Dropbox public folder due to Dropbox themselves limiting the API on any other folders.
The service has a few other handy tricks up its sleeve, like setting an expiration time for or customizing a link, tracking downloads, or even adding your own branding to shared links for a more professional impression. All in all, it's a simple, attractive, and easy-to-use service that adds much-needed functionality to king of file-sharing.