Lots of web sites have an option to export events as an ICS file that can then be imported into a desktop or web-based calendar (for example if you're invited to a Facebook event there will be an export button that gives you an ICS file).
By default in Firefox 3.5 on Vista I'm only offered three options here "Open with Windows Calendar", "Open with Other" or "Save File", so what I'd do is save the file onto my PC, and then log into Google Calendar and import that ICS file.
What I'd like is to have an "Open with Google Calendar" or "Import to Google Calendar" type option when I click the Export link.
I know that you can configure Firefox to send
mailto: links to GMail, and I've got the Operator extension installed that will detect hCalendar microformat entries embedded in the text and offer to import them to Google Calendar, but I can't work out how to configure either of these to do what I need.
Preferably I'd do this without installing any additional software, and in a way that will work with a Google Apps account (but if someone can point toward a solution for normal Google Calendar's that would be a good start).