One of my favorites is Google. All of these are available in mobile mode so that I can access them from my smart phone. I use Google Reader to keep up on my RSS feeds, Gmail to check email and my task list, Google Notebook and Docs to create and read notes and documents, Google Calendar and Google search. These applications allow me to stay organized, informed and in contact with people.
I also use Evernote, which is a great, free resource to take notes. What I really like is the ability to clip web sites to the notebook on Evernote. It is available as a desktop version, web version, and mobile version. The desktop version syncs with the online version so that you always have your notes with you. A very unique feature it has is the ability to search for text in a photo, which is very useful for me when I take a picture of a memo with my smart phone camera.
If you have some kind of web based email, it may have a calendar, tasks, and notes section to use. Microsoft Outlook Web access does, as does Google Mail and Yahoo mail.
Some other great online organizational services include: Remember the Milk, iPrioritize.com, pimonline.com, Jott, and Agenda at Once.
There are a lot of great resources and systems available. Check each one out and see which fits your needs and style the best. Look at ease of use, backup features, mobile access, and cost. Then choose one and get organized.