A few commands I use frequently but sometimes forget.
Concatenate mutliple PDF files
pdftk *.pdf cat output out.pdf
Convert a PDF page to an image
convert -density 150 input.pdf -quality 90 output.png
Batch resize images
mogrify -resize 640x480 *.{png|jpeg}
Crop an image
convert input.png -crop 0x4000 output.png
Create a GIF animation from images (10s delay, loop endlessly)
convert -delay 10 -loop 0 *.png animation.gif