BootPart is an easy tool for adding additional partitions to the Windows NT multi boot menu (for example, add the OS/2 boot manager or a Linux partition).

UNetbootin allows for the installation of various Linux/BSD distributions to a partition or USB drive, so it's no different from a standard install, only it doesn't need a CD. It can create a dual-boot install, or replace the existing OS entirely.

How to make GRUB load another operational system by default