Step 1: Make a Mess

I'll try to keep this fairly simple, giving you an idea of how to do some nice basics, but the rest should be left to your imagination. Be creative and try different variations out. (This is all Photoshop, btw)
Start of with an all black layer, then make another layer and use a soft white brush and paint a messy basic version of the shape you want.

Step 2: Ripple the Mess

Once you have the basic shape you want use Filter-Distort-Ripple. I usually do a large ripple and about 275%, but it's totally up to you, just do want you like. Also, after each ripple I like to Edit-Transform-Flip Horz or Vert and Ripple it 2 or 3 times, it just seems to look a bit better.

Step 3: Blur!

Now comes the fun effect! For this one I used Filter-Blur-Radial Blur. Turn it on zoom, instead of radial and pulll the "zoomed" graphic to the bottom, so its not just zooming from the center. This is another good thing to play around with, like putting it in the corner can give a "light from heaven" sorta look, or in the center, makes it more like your going into the distance.

Step 4: Color

If it's all ready and how you'd like it to be, merge the black layer and the effect (if you haven't already) and we can give it some color. Go Image-Adjustments-Hue/Saturation. Check the colorize box and make it whatever color you want with the Hue slider or the color range at the bottom. The Saturation slider gives it more or less of the color you picked.

Step 5: Be Creative!!

And now you have yourself a nifty effect ^_^
If you want to add something else behind it, set the layer mode from normal to screen and have fun. Also, if you want to give it an evil/dark kinda look, invert the layer(in the adjuments menu) and set the mode to multiply to cut out the light parts.