Thomas Walpuski reports that he wrote a patch to make OpenBSD isakpmd work with Linux 2.5 IPSEC. Furthermore, the main isakpmd CVS repository now contains this code! Some notes are on his page.
isakpmd is quite different from racoon mentioned above but many people like it. It can be found here. Read more about OpenBSD CVS here. Thomas also made a tarball available for those uncomfortable with CVS or patch.
Furthermore, there are patches to make the FreeS/WAN userspace tools work with the native Linux 2.5 IPSEC, you can find them here.