A.k.a boot manager, bootloader, boot loader.

GAG, the graphical boot manager.
rEFIt is a boot menu and maintenance toolkit for EFI-based machines like the Intel Macs.
Plop Boot Manager is a small program to boot different operating systems. The boot manager has a builtin ide cdrom and usb driver to access those hardware without the help/need of a bios.
XOSL
AiR-Boot is a x86 boot-loader that resides completely in track 0 of the harddrive. It‘s also independent of any OS, features a BIOS-like built-in setup, is localized to German, Dutch, French, Italian, Russian (including Cyrillic) and Swedish.
Smart BootManager is an os independent BootManager which has easy to use interface and many other features. The main goals of SBM are to be absolutely OS independent, flexible and full-featured. It has all of the features needed to boot a variety of OS.