Hal-test is a project mainly written in C, based on the MIT license.
an example of how to write good code that calls into HAL
a small example to figure out how HAL works, and how some code that used HAL was working (and how error checking around HAL calls should be structured)
NOTE: Fedora 16 has dropped HAL completely, so this won't work anymore at least in Fedora, but soon in probably other distros.