As part of a project to move my noisy Power Mac G4 to the basement (see my blog entery Silence is Golden), I recently bought a LinkSys WRT45GL. This is a 4-port ethernet and WiFi (802.11g) router which is hooked up to my DSL line and provides a firewall and port forwarding for the G4. I know that Kevin Heifner got this same router and immediately installed Thibor's HyperWRT version of the firmware. I don't use Vonage, and Skype has been working fine for me, so I didn't go so far as to replace the stock firmware (yet).
This router has the distinction of being built on top of a Linux kernel (interestingly, it was only after someone figured out that the firmware was based upon Linux that the firmware and technical details were released: the wikipedia for the WRT45GL).
I don't know if I'll ever replace the stock firmware with one of the open source versions, but it's nice to have the option. I doubt I'll try to do anything about it unless I run into a problem caused by the current firmware that isn't addressed by a stock updated from Cisco and is solved by some open source version with very high stability. I don't mind spending time setting these sorts of things up, but I do not want to maintain them on a regular basis.
By the way, I was amazed at the level of detail about the WRT45GL available from wikipedia. There's another blog topic for some other time.