I've used GNOME's default window manager for some years, but now I want to try a tiling window manager.
I want it to satisfy these two criteria:
WM must be lightweight
WM should not be complicated to configure
"Complicated to configure" varies greatly depending on what languages you're proficient in. XMonad was extraordinarily complicated for me to configure, but that was because I know absolutely no Haskell, and that's the language the configurations are in.
The two tiling window managers I've used and quite liked are:
Awesome. Awesome configurations are in Lua (as of awesome 3; before that they were in a custom syntax), but it's quite easy to configure and comes with a bunch of widgets; here's a screenshot of what my bar looked like at one point (there's also a graph widget, although I wasn't using it at the time):
The main reason I stopped using Awesome was the constant backwards-compatibility breaks; every point revision changed the API enough that I needed to spend days trying to fix my configuration file to work with it. It's possible that's stabilized more now
wmii, my current WM. Configurations by default are in shell scripts, but can be done any way you like as wmii exposes a 9p interface, which means you control it by reading and writing to files on a pseudo-filesystem. My current configuration is a shell script for the main configuration with a python utility script to do some of the work. The main downside is a lack of built-in widgets; it doesn't come with progress bars or graphs or icons. It's certainly "lightweight" though, which was one of your requirements
Personally, i3 takes the best features of the other big tiling-wm's (Xmonad, Awesome, DWM, etc) and combines it into one, Combined with dmenu/conky/dzen2 it's just what I look for in a WM. Check out the page; http://i3.zekjur.net/
There's a Arch Linux wiki entry comparing 13 different Tiling Window Managers, in grid-like fashion, here on the Arch Linux Wiki. Perhaps it would be hepful.
I haven't tried any of them yet, personally, but plan to in the near future when I have some time, so I'm following this thread closely as well.
I'd like to recommend two different tiling window managers, one dynamic and one manual.
XMonad is very powerful yet easy to learn, there is a short guided tour that explains its basic features and key bindings. It integrates smoothly with GNOME, the documentation is comprehensive and there are lots of additional extensions available. It supports the dynamic tiling paradigm, where windows are automatically positioned according to a selected dynamic layout. The downside, at least for some, is that XMonad is configured in Haskell and depends on the presence of a Haskell compiler.
i3 supports the manual tiling paradigm, where a screen acts like table divided into columns and cells. The user can freely rearrange windows, which allows for greater flexibility, but also requires more effort. In my opinion i3 feels modern in comparison to other tiling windows managers. It provides out of the box features that either require configuration or don't exist at all in other window managers, eg. maximize, urgency hooks, mouse resize. It's configurable with a simple plain-text file, which obviously is not Turing-complete. There's a nice video presenting i3 features that I suggest watching.
Of course both support multiple monitors without any problems and have a floating layer for applications that don't like to be tiled.
PS. One day Bluetile might be a good gateway drug for GNOME users, but it's still in an early phase of development.
I personally use Ratpoison when I need a light weight tiling WM - The configuration worked pretty well out of box, and since I'm quite adjusted to using GNU Screen for many years the leap to Ratpoison wasn't very difficult. I've also been using StumpWM Which has been more active in development than Ratposion.
There are a few out there, but there's one that sticks out (to me) which I found suited my needs:
The name is Qtile.
I've tried several: Awesome, Xmonad, i3, wmii, scrotwm and dwm.
dwm stuck with me for the following reasons:
I must admit it has some quirks:
But all in all, dwm has stuck with me for 1.5 years now.
I've been using wmii for quite some time. The configs are pretty easy to understand and you can use any language you want to further tweak the config you want.
ScrotWM It's a lightweight Tiling WM, inspired by Xmonad and DWM. You don't need to know any specific languages to tweak it, it's just plain text. Plus, it features Dmenu by default.