Here i will list light apps. I will not include cli apps. I will keep editing this. help make this list better.
Terminal
- xterm (installed on all distros)
- lxterminal
- mrxvt
- xfce-terminal
- mterminal
Web browser.
- midori
- arora
- conkeror
- Dooble
- reKonq
- epiphany
- dillo
- uzbl (if you can work out the vi-like controls)
- Kazehakase
Office
- Abiword (wordprocessor)
- GNUmeric (spreadsheet)
- scribus (desktop publishing)
- xpdf
- mupdf
- apvlv
- epdfviewer
- leafpad (text editor)
- gvim
- nano (really a cli tool but you’ll hardly notice)
Multimedia
- gnome-mplayer
- smplayer
- DeaDBeeF (foobar2000 like)
- Banshee (not particular lightwight – but my personal favorite)
- Kaffeine
- mpd + (quod libet, sonata, pympd, Xfmpc) but i don’t recommend this type of daemon/client setup as it never works right.
- Xfburm (disc burning)
IM
- empaphy (ubuntu default)
- emesene (MSN only)
- Sim-IM (Multi protocol)
- Ayttm (my personal favorite lighweight multi protocol IM)
- LostIRC
- WeeChat (irc)
Torrents
- Deluge
- transmission (Ubuntu default)
- Qbittoorent
Image Editors
- mtpaint
- Xpaint
- GNUpaint
Image Viewers
- feh
- ristretto
- mirage
Filemanager
- Thunar
- pcmanFM
- Rox-filer
Compression
- Squeeze
- Xarchive
- Xarchiver
Process monitors
- htop (ncurses but userfriendly)
- lxmonitor
WM/DE
I will not include tiling managers.
- Xfce4 (the most popular “lightweight” DE – currently sufferering from feature creep – closer to middle weight)
- fluxbox (I was a heavy user of this WM for a while)
- fvwm (currently my favorite lightweight WM)
- TWM (comes with Xorg – very featureless)
- Windowmaker (inspired by NEXTstep – i could never get hang of those boxes)
- JWM (used as default for DamnSmallLinux Puppy and Slitaz)
- IceWM (lightweight, used in the older eeePCs i think)
- LXDE
- openbox (you can replace with WM with the gnome default (metacity)) also works standalone.
For more check Arch Wiki List.