I believe all of OS neXt versions store each user's preferences for an application in 
$HOME/library  or /Users/~/library

The directory still exists. It's just made hard to find. Apple does a lot of that.

Lion seems to have added a leading dot making it .library and thus invisible.  It may have modified the Finder to make it invisible without the name change.

The details about which preferences are saved separately for each user and which are intended to be preferences for all users is just not clear.  Such things might be in a library associated with the /applications folder.

I believe it IS possible to install an application available to a user only by placing it in $HOME/applications but you might have to create the directory if it's not there.  It's entirely reasonable for two users to have two copies perhaps one being a different version than another. And, of course, if you write something yourself it ought to be in your HOME folder at least for a while.

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