At Nikhef, we use Indico as an organisation-wide agenda. It does have a (not advertised) option to export to some formats, including iCal. Google doesn't like the output, though. To be able to import these events into Google anyway, I wrote a little script to modify the output so that it does work. You're welcome to use the script too.
- RSS-to-iCal for importing RSS feeds into Google Calendar