Nanonet is lightweight network stack for Arduino aimed at small wirless radios like the nRF24L01 but supporting other interfaces including the ethernet shield.

Illustrative examples:

http://github.com/aaronds/nanonet/blob/master/NanoNet/examples/nano_example_ping_client/nano_example_ping_client.pde
http://github.com/aaronds/nanonet/blob/master/NanoNet/examples/nano_example_ping_server/nano_example_ping_server.pde

NanoNet - The core library (see examples folder)
NanoNrf24l01Interface - Interface for nRF24L01 (depends on http://github.com/aaronds/arduino-nrf24l01)
NanoEthernetInterface - Interface for the official Arduino ethernet shield (http://arduino.cc/en/Reference/Ethernet)
NanoEnc28j60Udp - Work in progress do not use.
NanoNetCronAction - Better timeouts (depends on http://github.com/aaronds/arduino-unix-time)

Zip files for extracting in Arduino libraries folder:

http://github.com/aaronds/nanonet/downloads