This is a lwIP-based network stack for pico]OS.
Device drivers are included for FreeBSD tap device and cs8900a chip on Olimex LPC-E2129 board.
There is also driver for Wifi modules using Broadcom Wiced SDK.
If this network stack is too big for your system, there is an alternate one called picoos-net, which is based on uIP (ripped off from contiki distribution).