Git is one of the most useful tools in my programming toolbox besides the compiler itself. Sure, there are holy wars over which version control system is best (Mercurial, anyone?), but if you’re coming from CVS or Subversion, Git is a huge improvement. Or will be, once you get over the learning curve. I highly recommend Pro Git as a way to become fluent in Git.
If you already use Git, however, you might be wondering how to make it more useful…