Tomato & DD-WRT allow you to set your own time outs for closed connections, allowing closed connections to be dropped quicker than by standard, which is quite helpful for P2P users. Both firmwares also allow for boosting the wireless transmitter power, increasing reception range. They also allow access to the b/g channels that the FCC doesn't allow to be used in the US, although this will only be helpful if you've got non-US standard client devices.
They also have much more robust logging and real-time monitoring capabilities compared to stock firmwares.
Moore's Law of Mad Science: Every eighteen months, the minimum IQ necessary to destroy the world drops by one point.