The Address Book (Ctrl+Shift+A) organizes your contacts, notify list, nick color overrides, and future ignore management in one window.
Tabs
Contacts:
Store notes about users with add/edit/remove.
Notify:
Manage your buddy list (synced with Notify List window).
Nick Colors:
Override display colors for specific nicks across the UI.
Control:
Ignore management (coming in a future update).
Opening
Press Ctrl+Shift+A, use the toolbar button, or go to Tools > Address Book.