<script>
function smiley(value) {
	var textarea = document.getElementById("form-name").textarea_name;
	var begin = textarea.value.substr(0, textarea.selectionStart);
    var end = textarea.value.substr(textarea.selectionEnd);    		
	textarea.value = begin + value + end;
} 
</script>
<form id="form-name" >
<textarea name="textarea_name"></textarea>
<input type="button" style="cursor:pointer" value=":)" onclick="javascript:smiley(':)');" />
<input type="button" style="cursor:pointer" value=":(" onclick="javascript:smiley(':(');" />
</form>