Pocket Informant is the best organizer existed on windows mobile, but now he won the title of best android. A huge plus: contains a lot of options, has a modest and compact enough for such functionality, the size is stable.

The feature set of Pocket Informant:

• Integration with Android calendar
• Easy backup function export / import all settings
• Synchronization with Toodledo
• Events and tasks can be displayed in different variants: for the month, week, day, Agenda, Task View
• Automatic deletion of completed tasks (on deletion or after certain period of time)
• plain list and grouped (by category, context, folder, action, status)
• Viewing tasks
• optional category (for events and tasks) and status (for tasks only) filters
• Ability to assign colors (for text and background) for each individual event or task, category, calendar, different task statuses, work and weekends, etc.
• Search and notifications integration with standard Android services
• Ability to show / hide different Android calendars
• Timebars and mini-text (event details displayed directly in calendar grid)
• Custom events and tasks (with ability to show / hide all event / task fields)
• Easily accessible commands for events or tasks directly from main views (assign category, move between calendars, move to a different date, context, folder etc.)
• Templates for events and tasks (including default templates for new items)
• highly configurable interface (with over 100 different options)