Let us recommend lightweight software alternatives to each other:

These are what I prefer, and why. I really hope that at least some of them are new to some people.

Compositing window manager
Unagi. XCB based.
http://projects.mini-dweeb.org/projects/unagi

Terminal emulator:
St terminal. Even lighter than urxvt
http://st.suckless.org/

Window manager
MCWM. XCB based, extremely lightweight.
http://hack.org/mc/hacks/mcwm/

Image viewer
gpicview. most lightweight image viewer I have found with GUI.
http://wiki.lxde.org/en/GPicView

Console Image viewer.
feh
http://feh.finalrewind.org/

File Manager
PCManFM. Not THE most lightweight, but still supports automatic mounting of USB devices, trash and GVFS in general as well as desktop backgrounds and icons.
http://wiki.lxde.org/en/PCManFM

Menu
yeganesh. "intelligent" dmenu that shows most used software first.
http://dmwit.com/yeganesh/

Panel.
Tint2. Most lightweight solution I have found that has both taskbar and systray.
http://code.google.com/p/tint2/

Know any better alternatives? Please post!

And of course there are tonnes of other software that I have not mentioned.

I am especially looking for the most lightweight alternative available for a text editor. Leafpad does not support syntax highlighting and gedit does not launch immediately, does not feel snappy enough :).