Openwrt inside a user mode linux.  Why would we even want this many ask?

There are potentially a lot of reasons, one obvious one to me, it allows
folks to 'kick the tires' without actually flashing up any hardware.  It's
also a great environment for porting over packages, you can get a package
fully functional in the uclibc root environment inside a uml without actually
disturbing your 'real router', and then rebuild for a specific target once
it's fully tested.

This is a first stab at a build that 'just works' and there will be more
cleanup to come.  The simple directions are:-

Configure for uml target
Configure with an ext2 root file system
build it all

In your bin directory you will find a kernel and an ext2 root file system
when it's finished.  Just run it like this:-

bin/openwrt-uml-2.6-vmlinux ubd0=bin/openwrt-uml-2.6-ext2.img

The uml will start, and bring up an xterm with the serial console in it.  No
networking is configured, but, it's a starting point.  The resulting file system
has just enough free space to start kicking the tires and playing in the world of
'embedded routers' along with all the resource restrictions that come with that
world.  

To configure networking and more, refer to the user mode linux documentation online

http://user-mode-linux.sourceforge.net/