WRT54G and WRT54GS projects 
These routers are nifty devices. From the factory, they run linux on a mips processor.
Version 5 runs vxworks tho, so you need a JTAG interface to get something else running on it.
Initially they come with standard wireless accesspoint features. Thanks to various projects, they can be modified to enhance performance or perform completely different functions.
My AP
On my AP device, I run Hyperwrt. It provides power adjustments, more control fields and some basic scripting abilities. Personally I use it to restart the dhcp deamon to point users to my actual gateway firewall, which is not the AP. This way I can run multiple dhcp servers to a single gateway despite the limitation of one server per actual device.
One of my future articles for MAKE Magazine covers one of my custom hardware modifications of a Linksys WRT54GS to be weatherproof and use commercial antennas.
Firmware
I tested SEVERAL different firmwares before choosing hyperwrt.
Openwrt is a complete linux distro for the routers. It's flexable, gives ssh access or whatever you want to install. It can be a pain due to various hardware configurations of the routers tho.
Linksysinfo.org maintains current links and info to the various software distributions and hardware configurations of the wrt54g(s).
Other projects
Add a serial port or two (it's already there, but you need to add a TTL-RS232 adapter and an actual port.
1-Wire File System Add 1-wire devices to a WRT54G.
Make a cool stand alone weather station using a 1-wire weather sensor and the serial port, the router can become a network interfaced weather reporter.
OLSR Mesh routed networking - Run mesh routing for large flexible (urban) network access projects.