Das U-Boot (the universal bootloader), is arguably the richest, most flexible, and most actively developed FOSS bootloader available. It's released under the GNU GPL and maintained at http://www.denx.de/wiki/U-Boot/.
If somebody writes a patch for the bootloader implementation of a particular device, you will find links to this on the wiki-page for that device. Additionally, we accumulate all the patches written for a particular bootloader on its own page. Hopefully, you can get a better comprehension of the functionality of the bootloader by having a look at them: