I'm a solo developer.
For short term, post-it notes and legal pads. There's something for me about having those TODO's constantly in my face. I always get them done. And it feels so good to crumple that ball of paper and throw it into the bin. Each sticky is 5-30 minutes of work.
For longer term/planning: Trello. When I'm ready to tackle something, I convert the Trello card into a sticky note, archive the card, and get to work!
Sometimes when I am feeling super unproductive/lazy, and have a buildup of sticky notes, I'll convert all those sticky notes into a neat 1 page legal pad page - handwritten, with checkboxes. By the time I'm done doing converting (and crumpling up all those sticky notes!) I feel rejuvenated enough to work on that second pass.
I love throwing stuff away. When I am SUPER unproductive, I'll tear away the blank part of the sticky note and throw that away.