I own several old WRT routers and they still can be used for having fun. The following is a list of resources I frequently use.
Must have reference to WRTRouters
How to Recover from a bad flash
The main resources site for Open WRT
Hardware list for Linksys WRT54G, WRT54GL and WRT54GS
An interesting forum post on howto for getting OpenVPN v2.0 up and running on OpenWRT.
Essential configurations post install for OpenWRT
A very interesting setup of an Internet Gateway: OpenVPN & OpenWRT on Asus WL-500G Premium
How to save yourself from disaster by Resetting the root password in OpenWrt