The rEFIt bootloader showing options for (the large icons, from left to right) Mac OS X, Linux, and Windows. There are also other tools below the OS selection in smaller icons.
rEFIt is a boot menu and maintenance toolkit for EFI-based machines like the Intel Macs. It can be used to boot multiple operating systems, including triple-boot setups with Boot Camp. It also provides a way to enter and explore the EFI pre-boot environment. The name "rEFIt" is likely a play on the terms "refit" and "EFI."
Development on "rEFIt" has been abandoned by Christoph Pfisterer in 2010, but it was forked in 2012 as rEFInd by Roderick W. Smith and continues active development.