<input type="button" value="Скрий" id="button" />
<script type="text/javascript">
var butonche = document.getElementById('button');
butonche.onclick = function(){
switch(this.value){
case 'Покажи': this.value = 'Скрий';break;
case 'Скрий': this.value = 'Покажи';break;
}
}
</script>