I have lusted after the Weller variable temp irons for a while now 
but couldn't justify the cost for my limited tinkering and repair.  I 
have 2 irons, a 25W one I use for circuit boards and a 40W one that 
works nice for automotive wire.

The problem is the 40W model just plain gets too hot and scorches my 
flux and degrades the tinning solder.  So, using some cross-platform 
technology from bee-keeping, i now have a variable temp iron.

In bee-keeping, we use a 'hot knife' for removing the wax cappings
from the frames of honey.  Same problem as the soldering irons-- the
hot knife gets too hot sorching the wax and honey, even tho it has a
crude on/off regulating thermostat.  I built a 110V rheostat that
allows me to regulate temps or speeds on a variety of devices. 
Essentially, it is a rotary lamp dimmer wired to a 110V receptacle
and placed on a line cord.  Plug the device into the outlet, set the
dimmer to whatever output you need and the rest is Duck Soup.

Here's what you need:
-metal 2-gang junction box.  ~$2
-600W dimmer.  $8
-110V outlet.  $0.69
-dual cover plate that has and opening for the 110v outlet and a wall 
switch. $1.
-3M rubber feet. $2
-old 3 wire extension cord.
-metal line cord clamp for junction boxes.
-a few wire nuts.

Wire the dimmer to control the 110v outlet.   cut off the female end
of the old extension cord and wire this to the dimmer. mount all
this in the junction box.   as the old saying goes, if you need more 
detail than this you shouldn't be trying it.  add the cover plate to 
the box and put rubber feet on the bottom of it.  now all i do is 
plug in this rheostat box to an outlet, plug my solder iron or hot 
knife into the controlled 110v receptacle, and crank to the best temp.

This allows me to have my iron on for extended periods without it 
turning to a black crust, yet have enough heat to really do a nice 
job.

-dan