The latest version is 0.5.3. (Older versions are also provided.)
The current development version can be found at the project's Savannah page. From there, it is also possible to syncronize with the current CVS.
John C. Toman of Compendium Technologies Inc. has made an alpha quality port of lwIP 0.5.3 to the Rabbit 2000 CPU with the Softools C compiler.
Florian Schulze has ported lwIP to DJGPP/MS-DOS and to Visual C++ 6.0/Win32. They can be downloaded here (DJGPP/MS-DOS) and here (Visual C++ 6.0/Win32).
Jani Monoses from Astechnix SRL has ported lwIP to RedHat's eCos. It is available here.
Guido Konrad's uC/OS-II port is also available from the mailing list archives.
Leon Woestenberg from Axon Digital Design B.V. has written a CS8900a network interface driver and is currently developing a DHCP client for lwIP. They can both be found here. The plan is to eventually integrate Leon's DHCP client in the main lwIP distribution.
Steve Reynolds of Citel Technologies Ltd. has donated his IGMPv2 implementation for lwIP to the community. It is available for download here (note that the copyright and license differs slightly from lwIP - read the license in the igmp.c file). The plan is to integrate his code into the main lwIP distribution.
Paul Sheer has incorporated lwIP into his PaulOS system and has written an alternative BSD socket layer. It is available for download here. It is copyright Paul Sheer and licensed under the GNU GPL.
UPDATED($Date: 2003/01/20 14:13:31 $)