there's a ton of freebies to try out. i have a ton of them myself. you never know what might work for a certain occasion. might as well hord all the freebies you can get.
Your best bet, is to do like I do, spend years downloaded and testing them all...I'm currently up to 986...and still collecting..."I Obsess with Excess".
vst's (virtual studio technology) are effect plugins and vsti's (virtual studio technology instrument) are instrument plugins. steinberg created the technology. other formats of plugins are dxi (cakewalk) and rtas (pro tools).