OpenWRT, and that's one of the reasons I've bought this router.
While being open is a good thing, being wide open maybe it's not.
This firmware has a backdoor, enabled by a simple utility (the windows version is available directly from the Netgear support site).
Once executed, it gives root access to the router without any authentication.
Consequences may vary from simple denial of service (you can reboot at will), or something more elaborate: download the /etc/shadow file, run John the Ripper, and get the admin password. Then, logging on with the web interface, you can flash a full version of OpenWRT, install tcpdump and capture all unencrypted traffic.
Problem is that the backdoor can be enabled by any device connected to the router, even via wireless; so be aware that allowing someone to simply use your connection, means giving them full root access.
The backdoor is not accessible from the Guest Network, so if you are going to allow someone to use your wireless, at least give them only the Guest Network.