My machine is seriously underpowered, and I think I need to start conserving every spare cycle. I know that my Gnome environment seems to underperform compared to my coworkers' KDE setups. But if I'm going to make that big of a switch, I might as well consider running something even lighter.
Is it possible to survive on a lightweight window manager and still run modern apps (Firefox, Eclipse, OpenOffice)? What's a good candidate window manager for me to try, and what do I need to know?
The window managers listed below all subscribe to the lightweight and fast approach. They are faster than fully fledged window managers like KDE or Gnome and trim down on most visual distractions. Which one you pick will be mostly determined by your own taste and what you can get to run.
There's a subfamily of these window managers, notably those which attempt to let you do everything by keyboard and let you tile your applications with minimal screen real estate waste. These can feel funny if you come from mouse-oriented window managers. XMonad and ratpoison are members of this family.
I'll second xfce, it's probably the most popular of the lightweight WM's out there (perhaps due to its inclusion in Xubuntu). I've also had good experiences with Fluxbox (it came with Damn Small Linux when I used that as a lightweight Linux VM (back when VMs were slow :-) ).
There is definitely an ease-of-use learning curve to reckon with when migrating to these more lightweight WMs, but the performance benefits aren't hard to see on older hardware (menus appear instantly, navigation is pretty snappy).
I used Fluxbox for a long time, which is great for people used to having windows floating around like in KDE, Gnome etc. It's pretty small, pretty fast and highly configurable, plus it doesn't look as ugly as some other "minimalist" window managers. ;)
A few weeks ago I switched to awesome because I like how efficiently it places and resizes my windows. It's perfect for me since I almost always have just a full screen terminal on one screen and a browser on another screen. It also supports mixed window styles, so you can have windows managed by awesome and floating windows on one screen (e.g. I have almost always a managed full screen urxvt open and a small floating mplayer window in one of the corners). It's as lightweight as fluxbox, if not even faster, but doesn't offer as many options for customizing the look and feel.
I like XMonad. It's very stable, has very low overheads, and has an active user/developer community.
XMonad is almost as minimal as ratpoison, but it displays multiple windows by tiling them, and even allows floating windows if you really need them (e.g. for modal dialogues or GIMP). It's certainly given my underpowered Ubuntu box a new lease of life!
Edit: I forgot to mention: XMonad is keyboard-based rather than point'n'grunt, so there's a bit of a learning curve, but once I got the hang of it I found that I was much more productive.
I am using fluxbox too. Compared to a desktop envionment, using only a window manager is not as convenient. You choose every component yourself which is both a strength and a weakness. ROX file manager and usbmount are great companions to fluxbox. Also take your time to find some dockapps that may be useful.
Icewm is quite nice and lean (used it for a while on an underpowered box but moved to KDE when the box was upgraded).
Enlightenment (v16) is actually very lightweight compared to gnome/kde these days, and it is very configurable (although, nothing seems to be as configurable as fvwm) Florian's suggestions are all good, but if you're used to gnome/kde, then you probably won't like ratpoison / xmonad.
icewm has done me good for several years. I don't need most of the crap that the big-time desktops offer, but i do like a clock and CPU usage monitor running in the bar along the bottom - icewm does have these. It is noticeably lighter in feel than the popular desktops. No weirdness such as tiled windows or anti-mouse attitude. Customizing the root menu is also easy, much easier than doing so in KDE or Gnome, which i never did figure out adequately. At one place i worked, the sysadmin saw my screen and decided to give it a try. AFIK, he's still using it.
I tried PekWM for some time. I really liked it. It allowed me to group programs of the same type, for example: Terminals.
The first thing you should would be to build your own kernel, with just the things you need. That will save tons of resources. Then, choose a lightweight WM. Ive found Enlightenment very light and awesome, give it a try. Later, you should look for lightweight replacements of the apps you use. You can replace OpenOffice with Abiword, Gnumeric. Just google, and you will find very nice alternatives to those ram-eater software. The thing I would recommend will be to avoid Java software, they'll run VERY slow on a low resources PC. Also, check for the services that are currently running on your PC, and disable the ones you don't use. Consider changing your current distro for a low resources distro. I found Debian very customizable and lightweight. Good Luck!