A project plan is the most important, yet often neglected artifact for the typical software development project. A project plan is a sufficiently complete, professional document for communicating information to software engineers and management needed to understand what the project entails, how it will be produced and controlled and what the effort and schedule estimates are for the project. The project risks, quality focus, and support needs are specified as well.
Key Components of a Project Plan:
Overview three to five paragraphs describing product function, platform, customers, schedule and development responsibility.
Goals and scope - what's in scope, what's out of scope?
Deliverables high level releases and content.
Risk Management identification of risks, mitigation strategies, how they will be managed.
Scheduling and estimates work breakdown structure, overall project schedule, resource allocation, estimation techniques used (justify estimates), how will project be tracked, how will schedule changes be made.
Measurements & Metrics what measurements will be collected, what metrics will be created, how will they be used, why were they chosen. These measurements will form the basis for the project's quality assurance plan and drive process improvement.
Technical Process what methodology will be used, what tools and techniques are required, what internal artifacts are required to be maintained. (This section typically references other development documents requirements, design, test plan, etc. The SDP identifies what artifacts are to be created and how they will be maintained.)
Rapid Development by Steve McConnell (available on Books 24x7) see Chapter Seven for background on software development life-cycle models.
Construx Software templates & examples (Steve McConnell's company) see "Engineering Management Section". Note that you will need to create a free login for this site. There are many very useful document templates and checklists available here.