Today, this just sort of clicked in my head.
1. Download Steam
2. Install and run Team Fortress 2
3. Go on an "achievement/idle" server
4. Leave Team Fortress 2 running overnight while you go to bed, or during the day while you go to work/school
5. Come back after 12 hours or so
6. Leave the server and open the crafting interface, then combine weapons in order to make scrap metal
7. If you have any paint or other miscellaneous items other than weapons, go on the Steam forums and find someone who will buy them for scrap/reclaimed/refined metal
8. Sell your scrap/reclaimed/refined metal for CE
You can only do this once a week, since it only gives you a limited number of items every week.
Thoughts?
The 'free' energy you get comes at the cost of a bigger electric bill for not turning the PC off.