Tiling window manager, very lightweight. It does its job, gets out of the way and doesn't try to push fancy stuff in my face, that's all I ask from a window manager (and more than what most of the others give me). Besides, I have my set of basic key bindings for it in muscle memory, so there's no point in changing.
[0] http://i3wm.org/
The tabbed windows looks very useful though.
I tend to have one main stack of tabbed windows on each workspace, out from which I "push" a window or two to create splits whenever I want to display multiple things at the same time.
I guess the ability to mix tabs with tiles (in whatever arrangements I want) is what keeps me using i3 instead of others. I'd love to get dwm's tags with i3's tabs and tile management.
A lot have the master window concept like XMonad which I do not like.
Not completely over the moon with i3 (some annoyances), but it's the best tiling WM I've come across to date; it's actively maintained, so should get better and better with time ;-)