How to make Conky text semitransparent?

To enable compositing, i.e., true transparency not just pseudo-transparency, you have to enable ARGB:

own_window yes
own_window_type normal
own_window_class conky-semi
own_window_hints undecorated,below,sticky,skip_taskbar,skip_pager
own_window_transparent yes
own_window_argb_visual yes
own_window_argb_value 128

If you want a semi-transparent black background, just set own_window_transparent no

To control the transparency level, play with own_window_argb_value, ranging from 0 to 255.

Source: http://crunchbanglinux.org/forums/topic/13231/solved-compositing-and-conky-wierdness/

This worked for me:

own_window yes
own_window_title conky
own_window_type normal
own_window_transparent yes
own_window_hints undecorated,below,sticky,skip_taskbar,skip_pager

Then go to the compiz setting (CCSM), enable "opacity, brightness and whatever", click on the leftmost "opacity" tab. Let Alt Button4 and Alt Button5 control the opacity/transparency setting.

After that just mouse-over conky and let the Alt button rock & roll.