There is a new free, open source app for Windows, Macintosh, Chrome (and soon, Android and iOS) for secured e-mail/messaging and file sharing called Peerio that looks promising:

https://peerio.com

For background on Peerio, see this Wired article by Andy Greenberg:

http://www.wired.com/2015/01/peerio-free-encryption-app/

If you'd like to try out Peerio (and help me evaluate it), feel free to add me as a contact. My Peerio ID is simply "1" (the numeral one).

Update: If you try Peerio, I advise against uploading your address book. Your contacts will receive invites to Peerio, which may not be what you want.

Update 2: Peerio has promptly updated the app to stop sending automated invites to uploaded contacts.