Development methodologies like agile can be applied in almost any context, including going "solo". You just have to take the best out of it and apply it to your needs.
For instance as a single programmer you can have a product backlog with your user stories, you can give them story points, plan your sprints, find your velocity, etc. In the end you'll be more organized and you'll improve your planning skills for sure. Hey, i'm not saying that you must have daily standup meetings with yourself! :)
This applies to all development methodologies/processes(RAD, DDD, TDD, ...) don't forget they're just ways of doing things.