Try Teamwork. We use it to manage a wide variety of projects, including SEO. One of the great things about Teamwork is the capacity to set up template task lists, with dependencies. For example, you might create a task list for on-page optimization and have default items on the list like Page Titles, Meta Descriptions and H tags. Within each of these task items, you could specify tasks and make some dependencies. For example, the person working on the project might need to edit the page titles an meta description of a page before moving on the H tags. You choose what you want to do. It's a paid tool, but well and truly worth the investment, especially if you are working with a team and need oversight of what everyone is doing.