If you go to "Marthapullen.com" & register you can then go to the forums & the Machine Embroidery forum. The members there list free designs sites each month. The downside of the site is they get allot of spam posts, especially on weekends....ignore any posts that don't have replies listed.
I have hundreds of free designs I've downloaded. If you want to do this you should set up a special folder & in that folder make sub-folders for each site you get designs from. It can get a tad confusing trying to remember where you got the designs & what type you downloaded. Copyright issues...most digitizers do not allow you to make items from free designs to sell.
I would say my fav site would be Embroidery Library. They offer free designs every month & many project ideas with instructions. Their designs are more dense that other sites are tho, so many would not be useful for thin fabrics, like quilt cottons, unless you use heavy stabilizers. You'd want to choose less dense designs for quilts.