The Linksys WRT54GL is limited to Wireless-G and therefore not fully up-to-date. But due to its robustness is still used as a development platform to tinker with alternative Linux-based firmwares.
Compared to the WRT54G, the WRT54GL has more RAM and Flash memory (although these values depend on the hardware revision (see http://en.wikipedia.org/wiki/Linksys_WRT54G for details) and therefore is a ideal platform for Linux based firmwares, e.g., DD-WRT (http://www.dd-wrt.com/). These firmwares enhance the router with additional functions that normally are not available in this price category.
In addition to the DD-WRT web surface that is provided by the DD-WRT firmware, it is also possible to install OpenVPN on the router.