:3 is a smiley-face, as many are quite aware. I keep getting people IM'ing me the smiley in laughter (and because they have no idea what it is). FYI: It is a mustache smiley-face. The "3" is a mustache
... :3
:{ and variations are mustaches.
:3 is indeed based on a cat's face and, despite being essentially western-style, tends to be popular with anime enthusiasts.