I just had a question on YouTube about where to find 3D printable terrain files and I realized that I've gone digging for this information myself without ever creating a useful list for myself. So I'm going to put this down here for now and then update it down the road.
Free Terrain Files
Thingiverse - This is my first stop for digging for any 3D files. It's not perfect as searching for something that you want can sometimes take all of your keyword-kung-fu to find something useful. Still, there is a lot of useful things there. If you want to save yourself a bit of time be sure to jump right over to Dutchmogul's designs. I don't believe that there's a more prolific 3D designer of game-related files on Thingiverse. Even the things that he makes for odd scales (like 18mm) are usually easily rescaled to make the more useful to whatever it is you are doing.
Yeggi.com - Unlike Thingiverse, Yeggi is really just a search engine for 3D files. If you search for 3D files on Google you are likely to run across results from Yeggi (which is how I discovered it). Most of the hits that you get will end up linking back to Thingiverse - but not all of them. I've discovered a lot of little sites that host 3D files with Yeggi. Additionally, you will sometimes get hits to Thingiverse that you don't get when searching that site directly.
Printablescenery.com - Printable Scenery is quickly building their brand to be THE place to find 3D printable terrain for gaming. Funding their business with Kickstarter campaigns, the site now offers a wide range of files for different game settings. Most of what they offer at the moment is either fantasy or "historic". The Historic range is a mix of things that seem mostly suited for WWII as well as a smattering of more eclectic pieces (like Marori huts or Native American structures). The Fantasy category the most packed with items as it includes both historic items that would work in a fantasy campaign as well as some true fantasy-inspired pieces like dungeon walls, strange towers, or the wonderful Winterdale line of castle walls and buildings.
Fat Dragon Games - FDG's offerings are all focused on dungeon tiles and fantasy dungeon setting related items. Of course some of the scatter terrain-like items could work in a variety of settings so even if fantasy and dungeons aren't your thing it's definitely worth a look.
Dicey Ventures - More fantasy offerings. In this case it's really stylized fantasy buildings like you might see in a 1980's cartoon. Interesting designs if that's what you're in to.
Thunder Chrome - TC's focus is post-apocalyptic shanty-town-style buildings. I really like their design aesthetic. It seems to be very Fallout inspired.
As I've researched this list I've found that many of the Kickstarters that offered 3D printable terrain never turned into ongoing companies. Some just vanished. I suppose that this is to be expected.
If you know of any individuals or companies that are offering downloadable game terrain pieces please either contact me directly or leave a comment.