What are some document management services/document version control applications out there?

Two great options that we recommend are Git and SVN.

Git: http://git-scm.com/
Git is a free and open source distributed version control system designed to handle everything from small projects to very large projects with speed and efficiency. It offers a variety of exclusive features such as cheap local branching, convenient staging areas, and multiple workflows.

SVN: http://subversion.apache.org/
Apache Subversion is a full-featured version control system originally designed to be a better CVS. Subversion has since expanded beyond its original goal of replacing CVS, but its basic model, design, and interface remain heavily influenced by that goal. Even today, Subversion should still feel very familiar to CVS users. For more information on what CVS is, refer to the following link: http://ximbiot.com/cvs/