Don't get me wrong, Google Calendar is a great app with a great UI, But there are some situations where you might want to access GCal from your desktop.  There are a couple ways of accessing GCal from your desktop using programs such as Rainlendar or Thundirbird.

 

GCal in Thunderbird: (works in Mozilla Sunbird as well)

Thunderbird is an outstanding app for handling e-mail but it could be better.  The one thing that Outlook has over Thunderbird is a calendar.  Not any more.  Mozilla has made an extension for Thunderbird called Lightning, which is an integrated calendar. Untill recently, i have been using Google Calendar for all of my calendar needs, but it would be nice to view my mail and calendar together without opening another window.  So, without further ado,

How to sync and edit GCal with Thunderbird:

Always online (most likely desktop computer)

Method 1:

1. Download and install Lightning Extension for Thunderbird
2. Download and install the Provider extension for Thunderbird
3. Go to Your GCal page then find your XML calendar feed (Settings –> calendar –> yourcalendar –> private XML URL) 
4. Open Thunderbird and click on "new" in the calendars tab on the bottom left
5. select "on the network" for calendar location
6. select "Google Calendar" and insert the URL you got in step 2
7. Choose a name and colour (doesn't really matter that much) then press "Finish"
8. Enjoy

Method 2:

1.  Download, install and start GCalDaemon
2. Follow same steps as method 1, except get the iCal URL and select the iCal calendar instead of Google Calendar if you have the provider extension installed, and replace "google.com" with "localhost:9090" in the iCal URL

Not always online/slow connection (laptops/dialup)

1. Download, install and start GCalDaemon
2. Run the password encoder and encode your Google password(located within the install directory in the "bin" folder") keep the window open for the next step
3. Edit the the following values in the gcal-daemon.cfg file (located in the "conf" folder) with your favorite text editor: "file.offline.enabled" from false to true, "file.enabled" from "false" to "true", "file.google.username" to your Google username, "file.google.password" to the encoded password you got in the previous step, "file.private.ical.url" to your calendar's private iCal URL minus the "http://" and "www.google.com" which should result in something like: "calendar/ical/username%40gmail.com/…"
4. In Thunderbird, start a new calendar and select the "on the network" option, press "next"
5. select the "iCalendar"option and input the path to the local iCal file (can be found by starting the standalone-start script - every line should start with "INFO|…"  if not, stop the service or close Thunderbird or both).  Make sure you use the exact file path from the standalone-start script - it will say "start listening to file …" where "…" is the path of the file.
6. Finish the setup process (colour and name - unimportant) and enjoy.

 

EDIT: (I had the old version installed by accident) Sync with Rainlendar (Windows & Linux only)

 Rainlendar is a nifty skinnable desktop calendar app that does exactly that - show a small calendar on your desktop. It can be configured to use the local iCal file created by GCaDaemon.

Old Version: 

1. Download, start and install GCalDaemon
2. See step 3 in previous section
3. Right click on any Rainlendar window and click on "config"
4. Click the "Plugins" Tab, highlight iCalendar plugin then click settings
5. Input the path of the local iCal file (found using the standalone-start script)
6. To see what the events are, right click on any Rainlendar window and make sure that "Visible" is ticked under the "Event list" Menu. (right click –> windows –> event List –> visible)

Current Version

1. Download, start and install GCalDaemon
2. See step 3 in previous section
3. Right click on any Rainlendar window and click on "Options"
4. Click the "Calendars" button in top bar and click "Add"
5. Input the path of the local iCal file (found using the standalone-start script)
6. To see what the events are, right click on any Rainlendar window and make sure that "Event List" is ticked off in the "Windows" menu. (right click –> windows –> event List)

 

Display with MS Outlook 2007

NOTE: This procedure will only display the contents of your Google Calendar and will not allow you to make changes

1. Open Outlook and go to tools –> account options and click the Internet calendars tab
2. Click "new" and input your GCal's private iCal URL straight from your calendar
3. enter any following information (pretty much unimportant)

IMPORTANT: If you are going to Sync more than one app to your Google Calendar using GCalDaemon, make sure that only one of them has write permissions.  Having more than that will probably have disastrous effects.

More tutorials on the GCalDaemon Usage page