The Namibian 24 hour Ultra is run by Across The Divide Expeditions. The race is split into an ultramarathon and a marathon. The marathon follows the same course as the ultra.
The race has been running since 2007. 9 people ran the first ever race, of which 8 finished. Year on year the race is doubling in numbers. 2008 saw 23 people start, of which 12 finished within 24 hours. 2009 saw 24 start and 18 finish. Bit more on Wikipedia
Here's a video of 2009/10...
Ultra - men: Tom Adams 14hrs 1min
Ultra - women: Allie O'Donovan 18hrs 11min
Marathon - men: Ryan Buck 6hrs 34min
Marathon - women: 5hrs 36min