If you rely on personal information managers (PIM) to keep you organized, then you'll find no shortage of open source options to choose from. They come in all shapes and sizes, from full-fledged applications that organize every facet of your life, down to small applications that help get you to the dentist on time.
One of the most well-known PIMs is Evolution. It provides integrated mail, a calendar, and an address book for the GNOME desktop. Evolution also features a to-do list, customized reminders, Web calendars, and supports multiple accounts. It's popular with Palm handheld device users for its syncing capabilities. Evolution has a large community of users, so finding support via mailing lists and IRC is typically easy.
Kontact provides similar functionality for KDE users. In addition to mail, calendar, contacts, and notes, Kontact includes KNode, a full-featured Usenet news reader. Kontact's data also can be synced to a variety of handheld devices, including mobile phones and PDAs using the included KitchenSync component.
Mozilla offers two types of PIM to choose from: Lightning and Sunbird. The former is a calendar that integrates directly into Mozilla's Thunderbird email client, while the latter is a standalone calendaring application. While neither app has a lot of bells and whistles, both are perfect for basic time management and calendaring tasks.
Outlook or Lotus Notes users will want to have a look at Task Coach, which is designed to pick up where the task managers of other PIMs leave off. Though not particularly feature-rich, Task Coach lets you create subtasks, view tasks as a list or a tree, assign them to categories, sort by attributes, drag and drop email messages and URLs onto tasks to become attachments, and more. Although Task Coach is only alpha software, its developers aim to make new releases available monthly.
TreePad Lite is a lightweight version of TreePad. Billed as a "personal database," TreePad Lite is a bit of a departure from traditional PIMs. Instead of focusing on appointments and to-do checklists, TreePad Lite manages all your information, including email, notes, documents, images, and Web sites. It even includes a native text editor.
If collaborative data management is what you're after, Citadel offers email, calendaring, and address books for groups. It's an extremely versatile package that can be used across multiple servers and domains, and also in conjunction with a combination of mailing lists, chat rooms, instant messengers, and RSS readers.
Chandler is another PIM designed for small group collaboration, as well as for "people who use their inbox as their task list." All data is managed from a central "dashboard" where items can be filed, addressed, acted upon, or set to appear as a reminder at a later date. Create notes, tasks, and events, move information across applications, manage folders, and publish items to group calendars, all from right within the program's easy-to-use interface.
Whether you're looking for a PIM to use on your personal computer or something on a larger scale for group collaboration, there are a myriad of open source data management tools to choose from.