there are several sites online that offer free quilt design software, but the only one that i am familiar with that you can use to design your own blocks as well as use block library to design quilt layout etc is ...
http://quiltassistant.com/free-quilt-design-software
it's a safe site, secure download. very useable.
am editing to say that i erred about block library. if you need free block libraries i have many bookmarked. my fave two are WideWorld of Quilts & Jinny Byers design board found on her website. both sites give multiple sizes for the blocks. another good site is quilters cache. let me know if you need links to these or other sites.