OpenWrt is a powerful distribution for open source routers. It supports for a lot of devices. A 2.6er kernel, an 2.4 is available, too. The web-interface is surprisingly useful, e.g. it supports switching between normal and advanced mode, in the advanced mode it supports more options and transactions for a set of configuration changes. It can be easily disabled. opkg is initially installed.
There is some documentation. They have a manual. Regarding hardware support the wiki has a lot of information. You can find a lot of useful stuff via a google search in their web forums (why can't they use mailinglists like a normal open source project?).
what distinguishes them, why pick this option
OpenWrt has a history of continuous development. It is not a fork, where you have to worry, if you get updates (i.e. current releases). ATM various firmware images of the current release for different hardware devices are available for flashing - no need to setup a cross-compile environment, figure out a sane default configuration etc. A lot of setups are supported out of the box (e.g. bridging, non-bridging, vlan tagging, pppoe, UMTS sticks etc.) - the web-interface is impressive - even if you don't plan to use it for regular stuff, it demonstrates powerful configuration possibilities of the base system.