Version control. You can observe the benefits even working alone on relatively small projects. And it's one of the pillars to experiment with more reliable ways of deploying software
I use git at work and for personal stuff. The learning path is maybe a bit steep if you come from things like Subversion, on the other hand I instructed a developer new to VCSs and so far he didn't complain, shoot his foot, or shoot mine :)
git is only my personal preference, generally speaking I recommend to go for a distributed version control system: it's modern, it's mainstream, and it's more versatile in terms of workflows you can implement.
