This is the Sonic Community Hacking Guide for the 2011 US/European PC version of Sonic Generations

Hacking Tools

QuickBMS

QuickBMS is used to extract files from Sonic Generations.
Primarily it is used to unpack .CPK files with this script (right click => save as).
However, it can also be used to unpack .ar.** files with this script. For editing purposes, it is best not to use QuickBMS to hack .ar.** files.

Crifilesystem has a tool used specifically for hacking Sonic Generations; CriPackedFileMaker.exe
While QuickBMS is the best tool for extracting .CPK files, CRI Packed File Maker is the best tool for repacking (or making, rather) .CPK files.
BEFORE REPACKING ANY .CPK FILES, you must first read this.

AR Tools is located within a .ZIP file inside this .RAR file.
It is one of two tools for editing .ar.** files (as in, files with a double extension of .ar and .00 - .99), among other file formats explained later.
To unpack a .ar.** file with AR Tools, one must simply drag and drop the .ar.00 file onto ar0unpack.exe. A folder will be created that contains the data of the original .ar.** file.
The folder will have the filename of the original file with .unpack at the end of it. For example, if the .ar.** file was named "Sonic.ar.00," the folder will be named "Sonic.ar.00.unpack."
Among these files will be a file named !archive-info.xml. DO NOT DELETE OR MOVE THIS FILE. This file contains the data of the filesystem of the original .ar.** file, which you NEED in order to re-pack the .ar.** file properly!
To re-pack .ar.** files, drag and drop the !archive-info.xml onto ar0create.exe. This will make a fresh .ar.** file with the same name as the original inside the folder with the !archive-info.xml (which SHOULD be the folder with everything else that was extracted from the .ar.**!)

Community member MainMemory has adapted his tool SADXsndSharp to work with Generations' .ar.** files, and said tool can be found in this link.
Generally this can be considered an easier way to edit .ar.** files, since individual files can be brought out for editing and put back in without having to make new .ar.** files.

More to come soon!