This setup is as simple as it is useful. It adds a nice 'glow' to highlights (in some cases called a "Bloom" effect IIRC) and bright parts of the screen/image.
Mind you that this effect is not exactly the same as the 'Specular' effect on materials. e.g. Here we can have a glow effect that reaches into the surrounding area. If you look close at the example render above you'll see a soft white glow around some edges of the model.
The node setup
In this setup we use a Colorramp to assign the color white to all bright areas in the rendered image. Everything else will be mapped to black (i.e has no effect later on in the Screen node).
Then we blur the resulting black&white image and overlay it with a Screen node onto the rendered image. Voila.
The Sharpen node is not needed to achieve the glow effect, but it reduces the general 'bluryness' after rendering with enabled OSA. You can just leave it out and connect the Render Layers node directly with the Screen node.
You can also use a different color than white to get a colored glow.
The example file
You can download the example setup used in the above sections here: Tutorials-Glow composition nodes.blend