Lilo very effectively boots FreeBSD. I'm currently running a dual boot with Slackware 12.2 and FreeBSD 7.2-amd64.
To set-up a dual boot with Slackware and FreeBSD:
1. Install Slackware first and leave a partition of un-allocated space for FreeBSD. I recommend at least 10 GB for FreeBSD.
2. Install FreeBSD onto the free partition. Do not install the FreeBSD boot loader. Lilo will boot FreeBSD for you.
3. Boot into Slackware and edit /etc/lilo.conf. This is my lilo.conf. I have a home partition mounted as sda3. FreeBSD is installed on sda4. Adjust your lilo.conf to your specifications, partition scheme.
# Linux bootable partition config begins
image = /boot/vmlinuz
root = /dev/sda2
label = Linux
# Linux bootable partition config ends
other = /dev/sda4
table = /dev/sda
label = FreeBSD
4. Run # lilo to update lilo.conf.
5. Now when you boot-up your computer you will have the option to select Slackware or FreeBSD as your OS.
6. Read the FreeBSD handbook for detailed FreeBSD installation instructions. FreeBSD Handbook