How can I create a “glow” around a rectangle with svg?

Here are some filters that provide different types of effect:

• Drop shadow (transparent black shadow with slight offset)
• Black glow (with a fixed colour)
• Object-coloured glow (takes the colour of the object to which it is a applied)

An example:

There’s a demo here.

The code:

<!-- a transparent grey drop-shadow that blends with the background colour -->
<filter id="shadow" width="1.5" height="1.5" x="-.25" y="-.25">
    <feGaussianBlur in="SourceAlpha" stdDeviation="2.5" result="blur"/>
    <feColorMatrix result="bluralpha" type="matrix" values=
            "1 0 0 0   0
             0 1 0 0   0
             0 0 1 0   0
             0 0 0 0.4 0 "/>
    <feOffset in="bluralpha" dx="3" dy="3" result="offsetBlur"/>
    <feMerge>
        <feMergeNode in="offsetBlur"/>
        <feMergeNode in="SourceGraphic"/>
    </feMerge>
</filter>

<!-- a transparent grey glow with no offset -->
<filter id="black-glow">
    <feColorMatrix type="matrix" values=
                "0 0 0 0   0
                 0 0 0 0   0
                 0 0 0 0   0
                 0 0 0 0.7 0"/>
    <feGaussianBlur stdDeviation="2.5" result="coloredBlur"/>
    <feMerge>
        <feMergeNode in="coloredBlur"/>
        <feMergeNode in="SourceGraphic"/>
    </feMerge>
</filter>

<!-- a transparent glow that takes on the colour of the object it's applied to -->
<filter id="glow">
    <feGaussianBlur stdDeviation="2.5" result="coloredBlur"/>
    <feMerge>
        <feMergeNode in="coloredBlur"/>
        <feMergeNode in="SourceGraphic"/>
    </feMerge>
</filter>