Well, in keeping with the old-school tastes I've had as of late, I've downloaded and tried out not one but two Quake clients - FuhQuake and Tenebrae.
Tenebrae is, at least in most ways, the better looking of the two clients. Two features - stencil shadows and per-pixel lighting - are Tenebrae's claim to fame and, for the most part, it looks pretty good. There is only so much you can do using comparatively primitive art and a modern engine, but textures have the same "sheen", for lack of a better term, that Doom 3 constantly overuses. The intro map as well as the first mission of episode 1 have had slight upgrades to the lighting and even some slight additions that really show off what the engine can do, but after that is where things start to get a little dicey. Every other level has not had it's lighting redone, and Tenebrae's new lighting technique serves to darken every level apart from the aforementioned two. That is... not so cool.
FuhQuake(I'm still waiting on Fuh Q2... heh) is much more simple in terms of scope - it's basically GLQuake with support for colored lighting and snazzier particle effects. It's also the one I prefer to play. Tenebrae is all well and good, but until every level is re-lit I'm going to stick with something I can actually see. Take a look at the screenshots for each one and see for yourself. One word of caution though - FuhQuake actually has Linux binaries available but if you want to play Tenebrae on Linux, you're going to have to get it though CVS and compile it yourself.