Aquariums in Fort Lauderdale, shelving in Boston, wood fencing in Omaha — free stuff is out there waiting to be picked up and put to good use. Free search and email alert service Ecofreek combs at least 45 sites for such freebies.
The full list of sources used by Ecofreek is listed on their FAQ/About page, and yes, it includes Craigslist. But Ecofreek also indexes a lot of swap services and regional portals that would be a pain to canvas on your own. You can expand your search to swap-style offerings, and if nothing shows up for an item you're hunting for, you can have Ecofreek email you when it comes up.
Ecofreek is free to use. Know of another free-friendly search or alert service? Tell us about it in the comments.