EXR.

 I have been trying to learn compositing, I'm getting there but slowly. I needed a few letters with a glow on a transparent background. When I tried to render and save the animation as .PNG files it wouldn't have it. Blender supports .EXR files and they will support anything you fancy popping in them. I've never used them before as I believed the files to be a silly size.

Works a treat, if you look carefully the background is crosshatched which means there isn't one. That is the Alpha Channel.

Here is a screen grab of the Compositor.

I have yellow highlighted the important bits. In the composite node make sure use alpha is checked and that the node is plugged in. File format has dozens of options but Open EXR is the one, make sure RGBA is checked and for this image Colour depth (or 'Z') can be popped on Half Float as that gives 16bits, more than enough for this, it helps keep file sizes reasonable. The files are surprisingly small at 3MB about 30% more than a PNG file but a fraction of a camera RAW file.

Now it's siesta time.