Zoho Planner is a category best calendar and planning system but there are worthy contenders. solo Technology has a good outline comparing Zoho Planner with "Getting Things Done", "Tracks", and "Vitalist". Even Google Notebook has a mention in this extensive outline of potential candidates.

However, I do not arrive at the same conclusion as Solo Technology. I still run everything online using a software service to house all my planning and calendaring needs -- Solo Technology is of the opinion that he is going back to paper and pen.

For me, Zoho Planner is the top of the category especially since it also maps into Zoho Writer, Zoho Sheet, Zoho Show, Zoho Wiki, Zoho Notebook, Zoho Projects, Zoho CRM, and Zoho Mail. AND just see what customers are saying about Zoho:

Zoho Writer is by far the best browser based word processing software available (and I've tried them all). It features auto-save, email support and the one no one else has: a spell checker. I will be telling everyone about this! Thank you Zoho for releasing me from Microsoft Word. -- Steven A Bristol

Watch some services charge for a more complex version and sometimes what you get for free is nothing like what you can get for free on another service. A lot of comparison shopping is required for this category. My money is on Zoho Planner.