Newly Updated/Optimized (2015) NRF24L01+ Library for Arduino, ATTiny, Due, and Raspberry Pi includes features/fixes from various forks and major updates RF24 Repo The same developer has created a few more useful libraries to simplify using a NRF24L01+ radio or even add new features to it.
The same maintainer has created an OSI Network Layer implementation using nRF24L01(+) radios driven by the newly optimized RF24 library fork. The library can be downloaded here.
Mesh network library named RF24Mesh, the library is currently in beta testing stage.
Build a mesh-enabled radio network between your sensors using the MySensors NRF24L01+ Arduino Library. The MySensors website also contains easy-to-follow build instructions and Arduino examples to help you create your own wireless sensors.
An Arduino port of the tinkerer.eu library. It works with the Sparkfun nRF24L01+ modules. Note: This library supports a small (but useful) subset of the features provided by the nRF24L01+ chip. Also see github repository
A very full-featured Library with support for many different radios, not just nRF24L01+: HERE
Also see this extensive nRF24L01 How-To on the ArduinoInfo Wiki HERE