There are many Linux HOW-TOs that deal with multiple operating systems on the same hard disk.
The Windows NT® Loader Hacking Guide provides good information on multibooting Windows NT, Windows® 95, and DOS with other operating systems.
And Hale Landis's “How It Works” document pack contains some good info on all sorts of disk geometry and booting related topics. You can find it at ftp://fission.dt.wdc.com/pub/otherdocs/pc_systems/how_it_works/allhiw.zip.
Finally, do not overlook FreeBSD's kernel documentation on the booting procedure, available in the kernel source distribution (it unpacks to /usr/src/sys/i386/boot/biosboot/README.386BSD.