A locally running version instance of issue tracking software. Use whatever you're comfortable with, but I go with JIRA; It costs $10 for a license for up to 10 people.
It helps me keep track of side projects. For a long time I had trouble remembering where to pick up a project and would get distracted by unimportant parts. JIRA helps keep that to a minimum and makes me feel more serious about them.
I even keep one project setup in it for personal stuff (renew passport, etc) just to keep all the TODOs in one spot, but whenever you tell people that they always make some wise-ass crack about using it to file a bug report about you personally...