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.
After a couple days using Peerio, I'm impressed. Feel free to use it to contact me privately if you like. Note that the e-mail address you use to register your Peerio account will be visible to other Peerio users you contact, so while Peerio offers good privacy, it doesn't necessarily offer anonymity.
This is my Peerio avatar. It serves to verify my cryptographic identity:
(https://antipolygraph.org/graphics/george-maschke-peerio.png)