A good number of apps, not MS, place folders in the Documents folder.
Appleworks, Eudora, Chronosync, Quicken, Roxio, to mention a few.
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If the software you installed uses Apples standard Installer application, then theres an easy way to see where it installed its files. First, find out the name of the installer. For example, the installer might be named WireTap Pro 1.1.1 Installer.pkg.
Now, open the folder located at /Library/Receipts/ , and find the installer in there. Strictly speaking, these arent copies of the installers, but just receipts telling you where it installed its stuff. To open these, youll need a fantastic piece of shareware software called Pacifist. Opening it with Pacifist will tell you exactly where it installed its files.
If the installer you used does not use Apples standard Installer application, then its a bit trickier to find the files. However, almost all Mac applications are good citizens, and place their support files in strict locations.
Applications that you install by drag-and-drop are the easiest to uninstall. First, delete the app, wherever you installed it by dragging. Then, go open up ~/Library/ (where ~ is your home folder), and look for any folder with the name of the application. For example, if theres a folder located at ~/Library/iComic/ , you can safely assume that this folder is used by the iComic application, and can safely be deleted without adverse affects to other applications.
You should also look inside the folder located at ~/Library/Application Support/ . If applications are friendlier, theyll place many of the preference files they create in this folder. Again, look for a folder with the applications name.
The other place you need to look is in ~/Library/Preferences/ . Usually, applications will have a preference file whose name is of the form com.companyname.productname.plist. Occasionally, youll have preference files whose names start with org or net instead of com this depends on the companys URL. For example, Mozilla comes from the Mozilla.org domain, and therefore its preference file will be named org.mozilla.mozilla.plist. Sometimes applications dont follow this standard, though, and they sometimes even put folders inside ~/Library/Preferences/ . Its probably just easiest to do a search for the applications name while limiting your search to inside ~/Library/Preferences/ .
If applications are meant to be accessed by all users, youll find their files in /Library/ instead of ~/Library/ . The difference is that the /Library/ folder, located at the top level of your hard drive, is accessible to all users. ~/Library/ is only accessible to the current user, with each user having its separate Library folder inside the home folder.
(Just to make it clear, ~/Library/ is shorthand for a user-specific library folder. For a user named simmy, ~/Library/ is the same as /Users/simmy/Library/ . For a user named sarah on the same computer, ~/Library/ is the same as /Users/sarah/Library/ . So ~/Library/ is not in the same location for each user. /Library/ refers to the folder named Library at the top level of your hard drive. /Users/simmy/ refers to the folder named simmy inside the folder Users which is at the top level of your hard drive.)
Installers can also install other plugins or files in specific places so that is accessible to Mac OS X. The location of where these things are installed depends on what the plugin or file is. If its a contextual menu, then you should look in ~/Library/Contextual Menu Items/ or /Library/Contextual Menu Items/ . If its a screen saver, look in ~/Library/Screen Savers/ or /Library/Screen Savers/ . If its a preference pane, look in ~/Library/PreferencePanes/ or /Library/PreferencePanes/ . You get the idea. Basically, you want to look in one of two places for files: first, look in the current users Library folder, and then look in the general Library folder.
Theres one other location of which you need to be aware. This is /System/Library/Extensions/ . Yes, this folder is located inside the System folder which third-party applications shouldnt touch (and you usually shouldnt either), but the /System/Library/Extensions/ folder is the sole exception to the rule (and this is actually the Apple-sanctioned place to put third-party system extensions). Since both Apple-created extensions AND third party-extensions go in this folder, youll need to be extra careful that you arent deleting required system resources.
Unless its obvious that the extension youre deleting is not Apple-created (for example, WireTap Pro from Ambrosia software installs an extension into /System/Library/Extensions/ called AmbrosiaAudioSupport.kext), Id suggest contacting the developer or looking for information on the developers website about the name of any extensions it installs. That way you can be absolutely sure youre not deleting an essential system component.
In general, when deleting files youre not sure about, its prudent to be safe. If you dont know whether you can safely delete a file or folder, just move it to your desktop, and restart your computer. If everything works fine for a few days or so and you havent noticed any adverse side effects, then you can probably delete the file. Again, however, if youre deleting something from /System/Library/Extensions/ , Id suggest getting info straight from the developer about what extension it installed.