Surge
I don't know. I can tell which router is right for /me/ - I currently use a Thomson Speedtouch (780WL). I like the fact that it provides both a web based GUI (for speed and ease of routine config) and a decent CLI for more in depth configuration. Thomson have a good guide to the CLI on-line at http://www.thomson-broadband.co.uk/codepages/content3.asp?c=7&ProductID=...
The router runs a linux kernel and busybox and unlike the BTHH (which is based on the Thomson) it isn't crippled. I also like D-LINK kit for home use. But if you really want the full flexibility of a linux firewall, then I'd go for IPcop. You /will/ need a separate box, but it has a very small footprint and will run on some pretty ancient kit (try 486 with 86Meg of RAM).
Alternatively, if you really want to get your hands dirty, go for openwrt (mentioned by Adam in a different post above).
Mick
What I really do care about is the hardcore access to the device. I want to be able to change things. I would like a real Linux system, with a real shell and iptables that I could tinker with, and something that expect would run on (tried expect on a telnet session to the BTHomeHub and things didn't work well). I'd also like this to be in one device, not having to use 2 power plugs for a modem and a separate router.
So, which router would be right for me?