Using Opera, follow the link from the Franz site to the GotoMeeting site. Then, set Opera to "Mask as Internet Explorer" in the "Edit Site Preferences" dialog that you can access by right-clicking on the background of the site you want to edit preferences for. After that, you may need to reload the GotoMeeting site (or maybe even clear your cookies and reload).
The above procedure should hopefully keep GotoMeeting from noticing that you're not using Internet Explorer. The same procedure can be used on other sites that complain about your using an "incompatible" browser.
Anyway, next you should be able to fill out the registration form and then you should see a button called "View Recorded Webinar". When you click that button in Opera, you should be prompted to save a certain long-named ".asx" file. Save it as any name you like. Let's say you saved it as "foo.asx".
Then you should be able to play that file directly in mplayer by typing:
mplayer -playlist foo.asx
Or, if you look inside that file (using cat or less) you should see the mms URL that I mentioned above.
That's it!
If any content providers happen to be reading this, please make your videos directly and simply downloadable as .mp4 or .avi or .wmv or .ogv videos, instead of making users jump through ridiculous hoops like this when they can't or don't want to use your streaming technology.
