dd-wrt is third-party firmware that can be loaded on many routers and it makes available many additional features such as advanced routing as well as a keep alive mechanism. It is maintained by BrainSlayer.
In the few days of using it, some advantages of dd-wrt are evident. It has been far easier to configure on my network of Linux and Windows computers, which use both static and DHCP IP addressing. The bundled Linksys software on the new WRT54G V8 device had long DNS lookup times on the Linux computers (probably needed to use the remote DNS resolvers instead of pointing to the Linksys box), for all lookups, at all times. But instead of re-configuring the Linux boxes, in the same amount of time, it was quite easy to install dd-wrt micro-edition using these instructions: How to Flash Linksys wrt54g
Still, dd-wrt may be useful only for the technical tinkerers. There may be problems which require digging through the details in the dw-wrt forums. There are problems of having the reboot command in some cases clean out all the settings! This now seems to be fixed by doing multiple hard-resets and reloading both the CFE and dd-wrt version v24 micro edition. There are performance problems that even after applying the suggestions on the dd-wrt wiki pages have not gone away. These only show up when using the wget network downloader on one of the computers, but reports suggest that programs like BitTorrent also cause similar issues with dd-wrt. The wget run takes around 5-10 minutes, and this causes dd-wrt to use up nearly all of its IP Connections (set to 1024), nearly all of them are IP connections in the TIME_WAIT state (which I understand has a 2 minute timeout). The impact is that other LAN computers may find DNS lookups to become extremely slow - take a minute or more, or even timeout. Even after applying suggested configuration changes, this problem continues.
But since the pluses seem more useful at this time than the minuses, and the same configuration that failed with the original Linksys software now works: /etc/resolv.conf still points to the Linksys box but there is no longer any delay in DNS lookups on both Linux and Windows boxes. There are other router firmware packages available, but for the new memory-limited Linksys wrt54g routers, only dd-wrt has a micro-edition that can fit on it. Even though this is the micro edition, it is still quite feature-rich.