#! (read as 'Crunchbang') Linux is fantastic for old/moderate hardware.
I obtained some old laptops that were being thrown away (had Windows installed making the system especially sluggish). I first tried Ubuntu (my main distro), but after being spoiled by Ubuntu on very modern hardware this laptop just felt too slow for me. I installed #! and the hardware immediately felt 5 years newer.
#! is based on Debian so you have plenty of software available (used to be Ubuntu based), and there are 2 versions, one with Openbox as the WM, and the other with XFCE. Of course you can have both WM installed (or any others you want for that matter) and switch between them freely if you so choose. I use the Openbox version, and when booted up the OS uses only about 50Mb of RAM.