Kehribar released a C-based portable nrf24L01+ library on GitHub:
I tried to make this library as portable as possible therefore, in order to use this library, you only need to define couple of GPIO functions in your project and that’s it! Moreover, this library supports auto-retransmission and auto-ack features of the nrf24L01+ modules.
nrf24L01+ modules can be found at incredibly cheap prices these days. Seeed and iTead sell those modules at around 3 USD but in ebay or aliexpress they can be found at around 1 USD.
Issue reports and comments for the library are welcome!
Via the forum.