AEtuts posted Form An Attractive Energy Ball by Stefan Surmabojov. This tutorial creates an abstract energy ball using Trapcode Form and looks at layer maps, color correcting, and some workflow tips.
Beyond this one, Andrew Kramer has several AE tutorials involving energy effects, for example a simpler energy ball (with just built-in effects) in 3D Light Casting, and fire effects might be repurposed for this effect in combination with other techniques. There must be another tutorial somewhere since this effect has been YouTube fodder for a while.
Update: here's the YouTube one by Alex Damon that got 2.4 million hits since February 2007. A revision was promised in October.