Linksys routers run Linux from firmware. Linksys supplies source code on its site; look for "GPL Code Center" under technical support.
There are several replacements for the WRT54G firmware. All add certain common features such as (a) the capability to ssh into the Linux running on the box, (b) European WiFi channels, and (c) VPN service.
Supports SNMP/mrtg. Said to have a good interface, convenient for home use.
Intended for Wireless ISPs, lots of stuff for routing and repeater operation. Open source, but you can buy support and private-release subscriptions. This outfit has been slammed for GPL noncompliance and apparently lost a lot of the good reputation it used to have.
A fork of the SveaSoft codebase from a few years back.
Workbench for people who want to experiment with their own customizations. Provides a framework and a set of modular packages supporting particular features.
Starts from the Linksys 3.01.3 firmware and adds a handful of features. Might be useful for those comfortable with the Linksys interface.
Another hacker's workbench, this one runs from RAMdisk so you don't have to reflash the box. Thus there's no chance of trashing your router. The disadvantage is that it has to be reloaded each time after you power-cycle.
Any of these can be installed using the firmware upgrade procedures.
Firmware for other Linksys hardware (notably the WAP54G) can be found here and here.
For a look at the techniques used to develop these firmware alternatives, there's an interesting site on hacking the Wrt54g by Seattle wireless.net.