The ChaoSaX-Tools will allow you to edit various parts of a Sonic ROM. Currently, they are mainly centered on Sonic 3 & Knuckles, but some tools work with sevaral Sonic games.

ChaoSaX-Zone 3K is a level editor for Sonic 3 & Knuckles. It allows you to edit all the levels, floor and sprite layouts, but it doesn't allow to edit the ring layout at the moment. This version is separated in several 'packs' (program, sprites, tiles). The version with installation program is required if you have tryed CSZ 3K and can't run it. Once it's installed, you will only need to download the upgrade when there are new versions. Also, the first time you download CSZ 3K, you will need to download Tiles and Sprites. Extract Tiles into /DATA/ART and Sprites into DATA/SPRITES. Sprites may change, so keep an eye on the version number of this pack. Just a last remark: CSZ 3K might work without the installation program, I mean, downloading the upgrade instead of the full pack.
CSZ 3K V0.30 with Installation program   Tiles   Sprites (0.1)   CSZ 3K V0.30 Upgrade

ChaoSaX-Stage 3K is a special stage / bonus stage editor for Sonic 3 & Knuckles. You will be able to edit the Sonic 3 special stages and the Sonic K slot machine bonus stage. Sonic K special stages will soon be added!
Version: 0.30

ChaoSaX-Pal is a palette editor for Sonic 1, 2, 2B, 3, K and 3K. It allows you to modify all the standard palettes (those with pointers). You will also see a description of each palette. Some palettes are still unknown though.

ChaoSaX-Extract will allow you to extract compressed data from a Sonic ROM. This include the SK special stages, S2 levels...

Chaos-Pack will allow you to recompress data decompressed by ChaoSaX-Extract!.
Version: 0.10

ChaoSaX-Locate is a very simple tool. It's not really an hacking tool. It will allow you to easily save hex locations you've found in a Sonic ROM. The advantage is that you can search for certain types of data, using up to 3 criteria.