MongoDB: Which C Driver?
The official MongoDB drivers are all to be found here:
http://www.mongodb.org/display/DOCS/Drivers
In addition, the official drivers all get moved under this root in Github:
That's the best way to get an up to date version of the driver. In terms of MongoDB and Ubuntu/Debian in general - the official packages are available as described here (Ubuntu repos have very old versions):
http://www.mongodb.org/display/DOCS/Ubuntu+and+Debian+packages
EDIT:
The driver you mentioned is this one:
https://github.com/algernon/libmongo-client
The driver maintainer mentions that it has "slightly different" goals from the main driver, but not what those are specifically. I think the main difference to look at is the level of maintenance and the frequency of updates:
https://github.com/algernon/libmongo-client/graphs/impact
https://github.com/mongodb/mongo-c-driver/graphs/impact
The official C driver is the basis for several of the other drivers, it is going to get a lot of developer time, quick bug fixes, immediate new feature support etc. The libmongo-client driver has not seen any activity in several months - I am not criticising here, just making the observation. For me, that alone would be a compelling reason why to use the official driver.