I would start with a question what exactly your problem is. We don't introduce project management methods for the sake of introducing them but to organize our projects better.
In environment you describe I believe the first problem is coordinating different projects and not managing any single one of them. If most of your projects are covered by only a single developer most of the time your project management effort should be focused on:
- planning, as that's what tells a developer what to do
- monitoring, as that's what tells you how well (or bad) you're going
- risk management, as that's what helps you to avoid issues
In one-man project either of those can be dealt with using very simple methods and rarely project management tools would help you much with them. Good plan isn't built thanks to a tool but is merely written down using one. Monitoring with one person can be as simple as daily water-cooler chit chat. Same with risk management.
From what I see the key challenge here is coordinating different projects/tasks among the team. A very simple solution which can help you here is Kanban. You can pretty easily visualize and organize tasks you have among different projects. You can adjust the method to the way you work. Even if you don't apply every principle, like limiting work in progress, Kanban should help you to coordinate team's effort.
And the best thing is it costs you nothing as long as you have whiteboard/cork board and a bunch of sticky notes. I assume here that the team is co-located. It you are distributed you will likely need software Kanban board, but that should still be pretty cheap solution.