For Plug-Ins there are quite a few...
Generally you must ensure they are for your version. If not they will either just fail to load or cause Maya to crash.
The default install path is <MayaInstallPath>\Bin\Plug-Ins, any mel scripts that accompany it should go into your Maya\mayaversionscripts folder.
For plugins that are part of programs like Syflex the install should take care of it.
Hope this helps.
Chris (formerly R@nSiD)
When the power of love overcomes the love of power the world will truely know peace - Jimmy Hendrix
Winner SM VFX Challenge 1
3rd Place SM SteamPunk Challenge (May 2007)