SeaMonkey
Several packages have their own private plug-ins which are best linked directly from the seamonkey "plugins" directory. These are noted in place, but it would be useful to have a cumulative list here for when SeaMonkey (or FireFox, or...) is installed later.
OpenOffice has one: libnpsoplugin.so -> <OpenOffice home>/program/libnpsoplugin.so
While of limited use (and currently requiring both proprietary blobs and libstdc++.so.5, thereby annoying EVERYBODY), the latest Linux version of RealPlayer (!RealPlayer10GOLD) also has two plug-ins: they are typically installed in the RealPlayer prefix, but symlinks are set up at installation time into any mozilla-y installations it can find.