SourceTree (https://www.sourcetreeapp.com/) is pretty good and free.

In case you don't mind going for a paid alternative, Git Tower is good too (https://www.git-tower.com/). Comes with 30 days trial.

You may also like to check out a list of GUI clients here:

https://git-scm.com/downloads/guis

I personally like Github Desktop for basic operations, it supports any GIT repo, not just GitHub. https://desktop.github.com

GitKraken looks pretty. I haven't used it enough to provide a full on recommendation yet though.

https://www.gitkraken.com/

GitX is my favorite, despite being quite buggy, because the others I've seen are all way too complicated.

I really just want visual log and visual add -p and GitX nails that.

Came across gitup today, looks interesting

http://gitup.co/

If you just want a very simple and opensource one, I use GitX: https://rowanj.github.io/gitx/

I've tried most of them and I keep coming back to SmartGit (http://syntevo.com).

Tower is excellent.

Tower (https://www.git-tower.com/)

My favorite tool, really simple one:

http://gitboxapp.com

not mac specific, but magit[1] is just extremely useful and fast/easy to use. Some times even if I'm not using emacs to code, I just open it to commit/work on git.

[1]: https://magit.vc/

vim/vimdiff -- just trolling, but it's what I use :)

Everyone else I work with uses: http://www.kaleidoscopeapp.com

I like SourceTree.