has some excellent free icons for prototyping and just fiddling around. However, the free pack is only available in black color.
I was googling for white ones and came across this excellent post on how to convert black icons into white using imagemagick.
Once I had my dusty old Ubuntu machine fired up, I installed imagemagick and converted this icon set to White icons.
Black Icon - Glyphish.
White ones - Custom converted.
These are posted on github for anyone else who may be interested.
All attributions / credit for this goes to makers of Glyphish.