PCB (terribly ambiguous name) does what you want. You open it, go to
View->Enable visible grid, View->Grid size->100 mil and you've got what you
want. It's gpl so there's no restrictions. The only downside is that it
doesn't have a windows binary available, though they do claim to have a
simple build script. I've never tried the build script because I'm on linux.
http://pcb.gpleda.org/P.S. Quick search turned up an older windows build:
http://sourceforge.net/projects/pcb/files/pcb/pcb-20091103/ . Not sure what
bugs have been fixed/features added since then. It's been fairly stable on
linux since about the 2007-2008 builds.
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