Lion Tweaks has been updated to version 2.0.1 to support Mountain Lion! Lion Tweaks is the popular software developed by Frederik Wiker and is available for download here. It allows you to tweak settings in Lion and Mountain Lion that were removed from Snow Leopard. Features such as: changing the iCal leather to aluminum, coloring the finder sidebar, and enabling airdrop on older hardware are all available using this product. You can also do things such as: show the user library folder, enable the 2D dock, and enable TRIM support on Lion.
EDIT: It seems that Lion Tweaks is not changing the skins on iCal/Calendar or Contacts/Address Book. Here is a quick fix for that.
If you have any questions about the tweaks listed in Lion Tweaks 2.0.1, please leave a comment below! Happy tweaking!