For Google calendar or Gmail accounts, there are 2 methods.

I) The first is the easiest, on this Calendar Page, click on the Button.
If you are already logged into your Google account, it will automatically add it, or it will prompt you to login before adding this calendar.

II) The second method can be done from your Google Calendar account.
  1. Login to Google Calendar
  2. On left hand side, under “Other calendars”, click the Add Link
  3. Click on “Add a public calendar”
  4. In the “Search Public Calendars”, type in “arizona hydrological society”
  5. It should be the first choice on the search results below
  6. Click on the “Add to Calendar” Button, and it should be added to your list of “other Calendars”
You can also add the calendar to Outlook 2007.
(Based on instructions found at the following website: http://office.microsoft.com/en-us/outlook/HA101674951033.aspx?pid=CH102499821033) You can subscribe to your calendar posted on Google Calendar. An Internet Calendar Subscription keeps your Outlook copy of your Google Calendar updated. Outlook periodically checks the Google Calendar for updates, and if any are found, they are downloaded to the calendar copy in Outlook.
  1. In Outlook, on the Tools menu, click Account Settings.
  2. On the Internet Calendars tab, click New.
  3. Paste the Internet Web address "http://www.google.com/calendar/ical/lkp46pshpig4s69b51s4lgqhu8%40group.calendar.google.com/public/basic.ics" in step 5, by pressing CTRL+V.
  4. Click Add.
  5. In the Folder Name box, type the name of the calendar as you want it to appear in Outlook, and then click OK.
If you have Outlook 2003, you can get it to work, but it is more complicated.

You have to install a 3rd party software.
Google has one: http://www.google.com/support/calendar/bin/answer.py?answer=89955&topic=13948 that converts the Gmail Calendar format to MS 2003 format and then it periodically syncs between the 2 calendars.