Somewhat customized Openbox. Six workspaces, everything keybound. I make heavy use of alt-drag and alt-middle-drag to move and resize windows without needing to find their targets. Control-fkeys for frequently opened apps like gnome-term, chromium, etc. Each workspace is a discrete task: writing code, testing, email/IM, etc. Helps keep me focused.

What I love about OB is that everything is configurable. A lot of WMs (ahem, metacity) give you no control over the basic operations because they can't imagine any other way to do it. If you find faster ways to do things, e.g. workspace warping with mousewheel, or reordering windows with win+middle click) OB actually lets you achieve them. It's all there in XML--and the defaults are well thought out, too.

I know several people who swear by xmonad, but I've never been able to deal with it for graphical work.