One of Mountain Lion's new features is Messages, which replaces the older iChat messaging system. To tell you the truth, I didn't like Messages that much at first. There wasn't really anything wrong with it, but it wasn't much fun to use.
More iChat Smileys
The problem was the lack of smileys. Messages came with some smileys, but nowhere near enough. And the smileys I used with iChat didn't work with Messages.
Fortunately, the creator of MoreiChatSmileys has released a new collection of smileys that work with Mountain Lion and Messages.
MoreiChatSmileys for Messages is still considered a beta product. It also lacks an installer, so you must install the smileys by hand. Aside from these little imperfections, MoreiChatSmileys for Messages works great. Suddenly, Messages is a lot more fun.
Installing MoreiChatSmileys for Messages isn't difficult.
Quit Messages, if it's open.
Download MoreiChatSmileys for Messages from the iSmileys web site.
The file will be downloaded to your Downloads folder, and is called StandardMoreIChatSmileys.smileypack.
Open a Finder window and navigate to the Messages app, located at /Applications.
Right-click the Messages icon and select "Show Package Contents" from the pop-up menu.
In the window that opens, double-click the Contents folder.
Within the Contents folder, there's a Plugins folder. Drag the StandardMoreIChatSmileys.smileypack folder from your Downloads folder to the Plugins folder.
Supply your Administrator password when asked.