Closed-Eye Visualizations, a form of phosphene.
http://en.wikipedia.org/wiki/Closed-eye_hallucination
I find it quite easy to induce the type of moving, cloud-like pattern shown as level 3 CEV on that Wiki page, and the fun part is trying to visualise it as 3-D and move "forward" into it.
I also see the same "moving light" as Sanctus describes, looking similar in effect to the level 3 image, except I'd describe it as starting small and dim from the right, then blooming and becoming larger and brighter as it moves fairly quickly towards the left. It's generally a deep purple with a lighter "hairy" and moving edge, brightening to a dull yellowish colour at its peak.
The final fade is quite interesting as there's usually a brief surge of brightness immediately before it vanishes and I always get the impression it "pops" out instead of fades out. It's almost like it implodes.
Curious how mine move from right to left, but Sanctus' move from left to right. 
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Giraffe: a ruminant with a view.