You can still get the newest DD-WRT builds even on the WRT54G.
That is a Wi-Fi router from 2002.
Of course, the features are sort of limited. I tried the VPN build (on WRT54GL), but I couldn't get the OpenVPN client to connect. I found some thread mentioning it may be missing something, but I don't know if that's the issue.
Anyway, without overclocking I'd expect like 2Mbps. I mean, it barely handles HTTPS. Just trying to load the WebUI maxes out the CPU for several seconds if trying to use HTTPS.
Perhaps it's not missing anything software-wise, but it's just so slow it times out during handshake.
So I just put the std build on it in case it will be useful at some point.