Android/iOS: Most messenger apps depend on both parties installing it on their phones. Blab is an exception. You can record a quick video message and send it to any contact via SMS, who can open it in a regular browser. Those who have the app can also reply.
Blab is available for free on both iOS and Android, but the latter is still in beta. The app shows your contact list for you to choose who to "blab" with. When you select your contact, don't lift your finger; the video starts recording while your finger is on the screen. Lifting your finger stops the recording. You can also drag your finger to an icon that switches between the front and rear cameras, as well as another icon to trash your recording. There is no limit on the time.
You can basically use Blab as a video walkie-talkie, if both people have it installed. Any Blab link can also be viewed on a PC web browser as an MP4 file.
There were no issues in the short time we used it, and it's only going to get better if more people adopt it. But even without that, you can at least send one-sided video messages in a few seconds.
Blab (Free) [Google Play Store]