http://www.spiralgraphics.biz/genetica.htm
Seriously overpowered software. Used it to create seamless textures (ofc.), but also animate a lot our ingame artwork.
In essence, it allows you to create/animate/transform artwork through functions applied on textures; combined with live preview. This made it a cinch to create a walking animation for our mech, which topdown was just a hyperbolic zoom function, combined with a sinus translation function for the feet, and the same, but in different direction for the chassis.