Other than GRASS, is there a free orthorectifier out there?

I’m looking to take aerial images that are approximately orthographic, and lay them over a DEM to get an orthoimage. However, I don’t really have the money for a professional Photogrammetry suite, like LPS.

I’m also using single images & DEMs, not stereo images, so E-Photo doesn’t quite cut it.

I think GRASS is the solution, but it’s not the most user friendly program out there.

Any ideas?