Tile tracker is a feature I have created for creating GIF animations of NES scenes.

It is available as a stand-alone program here:
http://bisqwit.iki.fi/source/animmerger.html

In the past, it used to be a direct patch to the emulators.

This was done because the emulator could directly give information regarding the background layers' scrolling position.

Now, as a stand-alone program, global scrolling has to be deduced algorithmically from the image contents. It is a bit slower, but it no longer depends on the emulator.

More information.