SpritesMind.Net

Here is a site with ways to compress NES graphics:
http://wiki.nesdev.com/w/index.php/Tile_compression

How about trying to port these over to the Sega Genesis? All these different formats are one of the reasons why I like the concept of tile compression on the NES and Sega Genesis.

These are very simple RLE compression format.
SGDK already include some RLE graphics compression (more suited for 4bpp graphics) and even some more elaborated ones. The problem is that unpacking code is not yet optimized...

Digital Pictures used a few different forms of compression in their games for the Sega CD and 32X that could be useful.

Descriptions (including some of my own ) can be found at this page on MultimediaWiki.