Git clients:
Cygwin + git (Windows): http://windows.github.com/ - git-scm.com (needed to also install ca-certificates into Cygwin - Gordon...)
git bash (Windows)
TortoiseGit (Windows): http://code.google.com/p/tortoisegit/
SmartGit (Windows)
SourceTree (Windows/OSX)
Tower (OSX)
Tutorials and other resources
Really easy tutorial which cover in a very clear fashion, the main Git commands: http://www.atlassian.com/git/tutorial/git-basics
Reference of "daily" commands: http://git-scm.com/docs/everyday
Git Real (interactive gamified Git tutorial) - http://www.codeschool.com/courses/git-real
Git Immersion (full length Git tutorial broken up into 'labs') - http://gitimmersion.com/
Try Git (interactive Git tutorial) - http://try.github.com/levels/1/challenges/1
When moving from SVN to git: http://git.or.cz/course/svn.html
If you want to learn the basics behind git you can try: http://pcottle.github.com/learnGitBranching/
Understand what is and how works the index, git add and git commit http://www.gitguys.com/topics/whats-the-deal-with-the-git-index/
On stashing: http://git-scm.com/book/en/Git-Tools-Stashing
On .gitignore: http://gitready.com/beginner/2009/01/19/ignoring-files.html
Pro Git (free online book) - http://git-scm.com/book
Git Reference - http://gitref.org/
A well written document to understand the difference between merge and rebase: http://blog.sourcetreeapp.com/2012/08/21/merge-or-rebase/
Good and illustrated article to understand the differences between merge, rebase and merge --squash: http://365git.tumblr.com/post/4364212086/git-merge-squash
Perhaps it explains a little and helps clear the confusion with git: https://dev.c-ware.de/confluence/display/PUBLIC/Contributing+to+Flexmojos