The Urgent & Important Matrix, also sometimes called the Eisenhower Matrix or Covey Matrix, is a method by which you can prioritize the things you have to do to take care of the most high-impact tasks first. The Joint Information Systems Committee has a short guide to the system which includes as part of a larger InfoKit information on time management techniques.
If you'd like a little help prioritizing your to-do list, consider Toodledo, a task manager that analyzes your entries based on date, priority, and task status. You can sign up for a free account and use it over the Web or via iOS, Android, and BlackBerry clients.
CNET: Toodledo helps manage your life in bite-size pieces.
Trello was created by Joel Spolsky (formally of Microsoft) and Michael Pryor, founders of Fog Creek Software. It's a free tool to help you manage collaborative projects. It's easy to use, and offers a simple drag-and-drop interface.
LifeHacker: Trello Makes Project Collaboration Simple and Kind of Enjoyable
ReadWrite: Trello: Online Collaboration Software at Its Finest
WorkFlowy is a very simple, easy-to-manage to-do list or outliner. You can customize its look and it works well in mobile browsers, too. While its simplicity and lack of flash may put some off, others prefer its down-to-earth functionality.
Wired's GeekMom: WorkFlowy is the Best ToDo List Browser App
LifeHacker: WorkFlowy Is a One List to Rule Them All Organization Tool
A good place to start learning about David Allen's famous method is to read his book, Getting Things Done, but even without that, there's plenty of information available online from Allen and others about the techniques and tools you can use to follow the GTD lifestyle.
Like Getting Things Done, the Pomodoro Technique is based on a book written by its creator, Francesco Cirillo. Although the venture has turned into a business of its own, you can download a PDF of the book for free and learn more about Cirillo's plan for time management.