Nirvana is a GTD task manager accessible from your desktop web browser and mobile devices.
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Pros
Pro Streamlined for Getting Things Done
Designed specifically for the Getting Things Done methodology as created by David Allen.
Pro One of only few faithful implementation of GTD approach
The Nirvana approach is one of the few that successfully encapsulates the spirit of Getting Things Done.
Pro Flexible structure for simple to complex usages and workflows
Pro Clean layout
It has a very simple, understated design.
Pro Useful history of actions
Nirvana offers a chronological look at completed tasks that is good for reporting and improving future estimations. That said, it could be improved by incorporating a true journal view rather than a simple list broken up by week.
Cons
Con No native OS X & Windows apps
There are currently no native apps for Windows or OS X, although the company has claimed to be working on them.
Con The Pro service tier still does not have common features
Common features such as notifications and sharing between applications aren't available, even in the Pro-tier. Its only native integration with other applications is adding tasks via a specialized email address generated for your account.