I have been playing with keybase and find it useful. If you are a Linux person, it is worth taking a look at now. If you use Windoze I suggest you wait a while as the Windoze port is very new.
So, what is it? The main problem it is designed to address is verification of your identity. Huh? Well, you establish yourself as a keybase user and then “tell it” about your other identities such as web sites and your twitter handle and it makes that information available in an authoritative way. But, beyond that, it offers secure file store and sharing.
Right now (and forever, actually) you get 10GB of encrypted file store which you can use as public or private store. You can share private files with others (ultimately other keybase users but you can share with someone based on their twitter handle, for example before they become a keybase user). The information is encrypted such that only the two of you (or five of you) have access. There is no key that the server has to access the data.
If you ever used Wuala, you will be familiar with the general idea. Your store appears within your filesystem. You can access it from the command line just like any other file space, using your file manager or even using drag and drop.
If this sounds interesting, I have a few invites available. Send me an email at fyl at fylz dot com and I will send you one. Note that I am fyl on keybase.