XBoard is the old classic. Probably quite functional for experienced users but not very friendly and needs to be retired.
eboard is the XBoard replacement (?). I would have assumed it's dead but checking the site it seems a GTK+ 2.0 version is about to be released. It probably has the functionality that I've been aiming for with glChess but not the simplicity.
Scid is a database and analytics tool for hard-core chess players. It hasn't been updated since 2004.
Chessmonk is a modern replacement for Scid. It's still in early development.
Gnome Chess is long dead.
glChess is the new Gnome chess...
Pychess is the other modern chess interface for Gnome. We talked about merging but their goals are more power-user where mine are more simple. They've just release 0.6.0 - will look at "borrowing" any useful code!
Knights is the KDE interface. Looks fine and plan to check compatibility with it.
Slibo is a 3D chess application for KDE. It seems to be dead but the board looks a lot nicer than the glChess one. So should look into salvaging materials, reflections, shadows etc.
Brutal Chess is in the early stages of development. I think they're aiming for mostly graphics.
Dream Chess has very nice full-screen 3D graphics. It's early in development.
So when you list them all the market looks quite busy. I guess I predict that the old hands will fade away and we will be left with chessmonk+pychess, glChess and Knights as the dominant programs. But you never know...
UPDATE: 17 Dec - Added Chessmonk which is the program pychess is working with