Slackware Linux comes with LILO, which in most cases is just fine.  However, if you want to boot an operating system on a GUID partition LILO won’t work.  GRUB will.  You can download a Slackware package of GRUB from here:  ftp://ftp.slackware.com/pub/slackware/slackware-current/extra/grub/grub-0.97-i486-8.tgz