* Who will I show X feature to?
* When will I show it to them?
* Can I show them a draft?
To me, the biggest risk of solo development is not how I manage a todo list, but that I'll build the wrong thing, because I waited to get feedback.
Some things that have helped me a lot...
* set up a time to show someone your progress on feature X before you feel totally ready to.
* ask someone to try and pick up and tweak some layer of your code (or pair with them)
* if you are developing a library--record hour long screencasts where you use it in a realistic way. Prioritize issues where you say, "oh, I should fix that...". Repeat but with another person driving.
I use github projects and a calendar (the calendar appointments feel more important!)