The Linksys WRT54G series of wireless routers, that includes variants: WRT54GS, WRT54GL, and WRTSL54GS, is an extremely popular among computer enthusiasts because of it's ability to run third-party open-source firmware such as DD-WRT and Tomato Firmware as it's firmware source code was released publicly under the GNU GPL. The first version was released in December 2002, but many different versions with hardware/firmware variations have subsequently been released. All feature two external antennas, 4 LAN ports, 1 WAN ports, and 802.11b/g support. The most recent version in the series is the WRT54GL, which returns to a Linux-based firmware, allowing for upgrades from third-party firmwares. It retails for around $65.
The popularity of this router is largely due to the ability to unlock advanced features, tweak power settings, and view advanced QoS statistics by running third-party firmware. By flashing the firmware, you void all warranties, and likely cannot return the router for any store refund if you inadvertently damage it.
Note: Most WRT54G and WRT54GS models (not WRT54GL) sold in stores right now are the v5.0+ variety running the VxWorks firmware instead of the Linux-based version and cannot be flashed.