While both calendars and address books are usually just features of email clients, Web browsers, or personal information managers, this wasn't always the case. They used to be worthy of stand-alone applications. Here are a few that still exist.
KOrganizer is the Calendar app for KDE, and also the calendar component for KDE's umbrella personal information manager (PIM), Kontact. The standard chronological views are available, along with searchable events and to-do items. I'm not quite sure why KOrganizer makes a distinction between the two. There is also the option to add time-relavent notes in the form of journal entries. The journal is searchable, just like events or the to-do list.
The interface follows the new KDE style with multiple panes, showing as much information on the screen as possible, though calendar apps in particular have been using that design for many years. The journal feature makes up for the seemingly useless differentiation between to-do items and events. And potential integration into the powerful Kontact PIM make KOrganizer a little bit more attractive than Sunbird, especially if you run KDE.
Sunbird is a stand-alone calendar application from Mozilla. Sunbird lets you build separate calendars and manage them as one. For example, you could build a personal and a work calendar so that you could view appointments separately, yet still be able to see all your appointments in another view. The typical day, week, month, and upcoming event views are available, as are repeating events and reminders. You can also publish your calendar to a Web site, but with so many comparable Web-based calendars available (all editable via a site), why bother? Sunbird is a pretty solid and straightforward stand-alone app, even if the utility of such a piece of software is in question.
As you may have guessed, KAddressBook is the KDE address book application. While this is a standalone app, it integrates into Kontact to fulfill the address book function.
With KAddressBook, you can create multiple address books, each with their own contact and distribution lists (AKA groups). As far as contact information goes, KAddressBook is pretty complete. There is a lot of information to enter about your contacts, spread over a ton of different fields. At first, the information fields might seem like overkill, but they are all searchable. This means you can pore through your contacts, even if you can only remember small tidbits of information, like a partial name, birthday, or hometown. KAddressBook is a capable application, even for those of us with a horde of contacts.
Rubrica is an address book for GNOME. This app makes plain use of the Rolodex metaphor in its organization of contacts.
You can create several address books, which are made up of cards. Each contact is given a card that contains many fields of information about that contact. Rubrica uses categories or groups like, friends, relatives, office, and work to differentiate between how a contact relates to you. In this way, you could keep one large address book instead of multiple smaller ones, because Rubrica lets you show contacts by group using the left-hand pane.
If you decide to create multiple address books, additional tabs appear in the AddressBooks pane. Rubrica is on par with KAddressBook. The decision to use one or the other really just boils down to your preference between GNOME or KDE.