Mosaic software?

I am looking for software that will work under Leopard that will create mosaics. The only requirement I have is that I can export the image into a good enough format so that I can print it on poster (maybe 20x30) at Kinkos or something. I have tried MojoMosaic, a free download on the Apple site, but it crashes without actually working. I appreciate any and all help

Thanks! I really like the 2nd program you mentioned.

Now say I want to print this image 20x30 do I need to start the mosaic over and calculate a certain resolution before it looks good for 20x30? My base image is 1600x900ish.