Are there any comprehensive open-source alternatives to Matlab's Image Processing Toolkit?
Octave's image And for Scilab there is SIP and SIVP
Both Octave and Scilab are very Matlab like, but I'm not sure how similar the image toolkits are.
Also look into OpenCV if you are comfortable with C/C++ or Python
Try ImageJ (also read: http://en.wikipedia.org/wiki/ImageJ)