iStat Menus uses MenuCracker to enable the use of 3rd party menu extras. MenuCracker is released as open source under the Artisic License.
The source code for MenuCracker is available for download as part of MenuMeters by Alex Harper. http://sourceforge.net/projects/menucracker/
As of iStat Menus 3.17, we also include a modified version of MenuCracker that is compatible with OS X 10.7. The source code for this version can be downloaded here: http://www.bjango.com/istatmenus/menucracker_lion.zip