Homeschool Share (HSS) is a on-line cooperative effort of several homeschooling moms to provide free but quality literature-based unit studies and resources. They also have lapbooks. This site is nicely broken down by grade and you can search by author, book title, Caldecott Award winners, geography, holiday, season and subject & lesson index. The lapbooks are broken down by grade level and there is a complete section on animal studies. They also have a forum for discussions too.
Amanda Bennett's Unit Studies offers a variety of unit studies for sale on her sight. She also has lots of great tips on how to use unit studies in your homeschool. She also offers a free unit studies newsletter you can subscribe to and a blog you can follow.
The Homeschool Mom offers a list of unit studies but these are linked to other sites and some are just lesson plans. It's not quite as good as the above sites but it still offers some good resources.
Cindy Downes is a veteran homeschool mom who educated her two children at home from preschool through twelfth grade. She homeschooled from 1981 through 1985 in Delaware, 1985 through 1988 in Virginia, from 1988 through 1991 in Pennsylvania, and from 1991 through 1994 in Oklahoma where her children graduated.
This website is the result of that adventure. She has including everything she learned when teaching her own two children, as well as what she learned from consulting with others who dealt with problems such teaching multiple children at multiple grade levels, teaching older children while taking care of infants and toddlers, teaching Visual, Kinesthetic and Auditory Learners, and teaching teens who want to start a business or go into the ministry rather than attend college. In addition, she continually adds free unit studies as she writes them. Her goal is to keep this website up-to-date with the best curriculum she can find that will help parents of children who are Visual, Kinesthetic, and Auditory Learners or who have one or more of the other problems listed above. Cindy has lots of great homeschooling information on her website.
These are just a few but hopefully they will help you get started if unit studies is something that might be a good fit for your family. A great place to purchase unit studies is at Currclick - they carry Amanda Bennett Unit studies and more.