Rainlendar is a customizable calendar that displays the current month. It is a very lightweight application that doesn't use much system resources or take much space on your desktop.

The manual for Rainlendar can be found from here (html) and here (pdf).

Features

Small and lightweight
Different type events can have different appearence
Supports Windows transparency
Synchronizes events between several clients
Localized for multiple languages
Shows an alarm when an event is due
Supports iCal files.
Shows Outlook's appointments
Includes a todo-list
Works also as Litestep plugin
Displays the current date in tray icon
Can stick to the desktop (i.e. doesn't hide with Show Desktop)
Hotkeys for quick access
Easy skinning with an UI
Can be controlled from 3rd party applications with !bangs
and more...