For development of my OpenBSD snapshot tracking utilities I used RCS. See
rcs(1)
More than two years ago I decided to play with
git, one of the newer kids on the 'revision control' block.
Having forgotten most of it, I happened to find a reference to a free online book about git:
http://git-scm.com/book this morning. A dead tree version of the book is published by
http://www.apress.com/
Having read only a few chapters, this post is not a review, but I found this book a nice addition to the
/usr/local/share/doc/git/user-manual.html that is part of my git install on OpenBSD.
If you are interested in git and want to learn more, this free online book surely is worth reading.