Any of the trapcode plugins are absolutely great. They have demos available on the site: www.trapcode.com
DigiEffects also makes some good plugin packs. Delerium is probably my favorite of theirs. Once again i think they have a demo of it on the site:
http://www.digieffects.com/
I just checked out Trapcodes site. It appears that there are some more affordable (less than $100.00) third-party plug-ins available for After Effects. (I feel like an ass).
Most All plugins are too exspensive to justify in AMV use. If you were making money then I could see shelling out the $2000 for software. But just for AMVs, no I don't think so.