There are many Linux HOW-TOs that deal with multiple operating systems on the same hard disk.
The Linux+DOS+Win95+OS2 mini-HOWTO offers help on configuring the OS/2® boot manager, and the Linux+FreeBSD mini-HOWTO might be interesting as well. The Linux-HOWTO is also helpful.
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.
This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.
For questions about FreeBSD, read the documentation before contacting <questions@FreeBSD.org>.
For questions about this documentation, e-mail <doc@FreeBSD.org>.