Both ImageMagicK and GraphicsMagicK have command line tools and a Perl interfaces. GraphicsMagicK is a little newer with all that implies.

ImageMagick

Command line tools for converting images are found here.

convert -background transparent -density 300 -quality 100 test.pdf test.png

GraphicsMagick

Does a lot of what ImageMagicK does. Command line tools for converting images are found here.

gm convert -trim -transparent white -background transparent \
-density 800x800 -resize 25% test.pdf test.png