According to Wikipedia, "Webcal is an unofficial Uniform Resource Identifier scheme for accessing iCalendar files. Webcal was initiated for use with the Apple iCal application and has become a common de facto standard for accessing iCalendar formatted files via WebDAV. It is not an official URI scheme, such as http and ftp, as registered with IANA. The Webcal protocol prefix is used to trigger an external protocol handler which is passed the URL of the .ics file rather than being passed the downloaded contents of the file, in much the same way feed is sometimes used to trigger external RSS readers. The idea is that with this protocol prefix the target file should be subscribed to rather than imported into the calendar application as would happen with a simple download."
Here's an example of webcal: link that lets you subscribe to a calendar that includes the US holidays. You can find more calendars at iCalWorld and iCalShare.
To edit the protocol handlers in Google Chrome, open a new tab and paste in the address bar: chrome://settings/handlers (you can also go to Options, select Under the hood, click Content settings and then click Manage handlers).
Firefox also has this feature and it's even more powerful because it also works for filetypes. Unfortunately, you can only associate them with native apps, not with Web apps.
{ Thanks, Sterling. }