Wikr is what I use. Right now it's only available as an iOS and Android app. You specify how long you want your messages to exist for and the countdown starts when the receiving party views the message. Slightly clunky, but very very secure:
From the website:
App:
ID and device info are cryptographically hashed with multiple rounds of salted cryptographic hashing using SHA256.
Data at rest and in transit is encrypted with AES256.
No password or Password hashes leave device.
Messages and media are forensically wiped after they expire.
Server:
In contact with encrypted messages/media only.
Never in contact with passwords of private encryption keys.
Deletes messages on delivery.
Interacts with only hashed ID and device info.