well, honorable forum member Dan Slagle has a list on his website... but
http://danslagle.dvmix.com/mac/iMovie/links.shtml
… most "free" plug-ins are.. "antique", for sure, not universal...
but affordable are the plug-ins from
http://www.imovieplugins.com/
Christian, author if the plug-ins, offers a risk free trial.. and 1.50 - 5$... that's almost free... 😉
and just to tease you, what you can accomplish by spending some money, have a look at geethree.com... 😉
or even better at apple.com, Final Cut Express...