For those looking for a Thingiverse alternative, or that are interested in getting paid for some of your designs (ahem, @Walter_Hsiao ), check out Pinshape.com. You can find plenty of free things to print there too, search for objects by printer used, and sometimes even find the settings used for printing by users that have actually printed the objects. I’ve been checking it out for a few months and it’s pretty neat, and also independent from any specific platform or company.
Another great resource for searching for things to print is http://Yeggi.com It searches many file repositories at once, so if you’re looking for something that’s not on Thingiverse, you might be able to find it through a Yeggi search on another site.
http://Pinshape.com