Mac: SendRail is possibly the simplest file sharing app we've come across, and we've seen our fair share of ways to send files online. It connects to your Facebook and Gmail accounts so that you can click a file on your Mac, press a command key, type a name, and know that your file will get to your friend as quickly as possible.
The Good Stuff
The video above explains how SendRail works in detail, but here's the gist. You install the app, connect it to Facebook, and add Gmail as well if you want. When you want to send files or folders, click what you want in the Finder and press Command+U. There is no size limit, so you can send as much as you want.
You'll be prompted to enter a friend's name. Do that, press return, and the file will be sent behind the scenes. It's incredibly simple to use and is about the quickest way to send files that we've seen.
The Bad Stuff
That said, SendRail's simplicity also creates a number of problems. Once you initiate the file transfer you don't actually have any evidence that the transfer is happening. In my tests, everything arrived at its destination but I never saw progress or proof unless I went onto SendRail's site to check that the file arrived. Progress could easily be indicated in the menubar where SendRail lives.
Additionally, typing a contact's name doesn't necessarily tell you where the file is being sent. If you've connected only Facebook, you know the recipient will get a Facebook message. If you have connected Gmail as well, you have to type their name and then [GMail] as well. SendRail will autocomplete everything for you, but there's no indication that you need to do this in order to send to Gmail. If you have multiple addresses for a single person, it's not clear which one SendRail will use. You also can't type an email address of someone who isn't a contact and send it to them. They have to exist in your account already or you can't send them a file.
The Bottom Line
While these are most certainly drawbacks to the service, once you know how SendRail works they're not that big a deal. As the service is free and also really useful for Facebook and Gmail users running OS X, it's definitely worth a look.