Mostly current but needs to be updated to include bump maps and metal maps.
This thread can also be of interest http://www.mecabricks.com/en/forum/topic/128
This table contains the hexadecimal values for all the materials available in Mecabricks as well as the conversion from other systems like LDD or Bricklink.
wlange has also created swatch libraries for photoshop and illustrator: http://www.mediafire.com/file/tnfoui8jxkra6f7/Mecabricks+Colors.zip
UmbraManis did the same for gimp and inkscape: http://www.bricksafe.com/files/Umbra-Manis/OpenLDDFiles/Mecabricks.gpl
This folder contains all the UV maps available for Mecabricks. Please make sure that the decorations you create are of the same size as the corresponding uv map.
I also wrote a guide about uv maps for minifigure decorations https://www.mecabricks.com/en/forum/topic/1357
[Update] Icons are now automatically generated on the server without the need to use a third party software.
To submit you decorations, please use the Drop Box section of Mecabricks. Guidelines are also provided in this post.