200byteDemoChallenge!

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FischGurke

Amazing programs so far...
Here's my attempt... 255 bytes in size. :)

g=300;Graphics g,g,0;Local a[g*g];Repeat;a[Rand(g)]=1;For x=g To g*g
If(x+2)Mod g=0 Then x:+3
f=Rand(-3,2);If a[x]~a[x-g+f]
a[x-g+f]=0;a[x]=1;c=255;SetColor c,c,c;Plot x Mod g,x/g;SetColor 0,0,0;Plot x Mod g+f,x/g-1;End If;Next;Flip 0;Until KeyDown(27)

You can change the size of the graphics window with the variable g. Any optimisations are welcome! :)
Here's the "clean", commented code:

g=300
Graphics g,g,0 'square graphics window
Local a[g*g] 'initialise an array to hold the "snowflakes"
Repeat
	a[Rand(g)]=1 'spawn a snow flake
	For x=g To g*g 'update all snow flakes
		If(x+2)Mod g=0 Then x:+3  'skip some pixels if we're close to the edge to avoid "stuck" flakes
		f=Rand(-3,2)  'move flakes to the left or right randomly
		If a[x]~a[x-g+f]  'if the pixel the flake tries to move to is unoccupied
			a[x-g+f]=0    'move flake by swapping values
			a[x]=1
			c=255   
			SetColor c,c,c  'set color to white
			Plot x Mod g,x/g  'draw flake
			SetColor 0,0,0  
			Plot x Mod g+f,x/g-1  'remove trail
		End If
	Next
	Flip 0  'woo :)
Until KeyDown(27) 'repeat until escape is pressed