Cita: <html>
<head>
<script>
function sel(c){
formu=document.forms['formulario'];
caracteres=c.length;
if(caracteres!=0){
for (x=0;x<formu['lista'].options.length;x++){
if(formu['lista'].options[x].value.slice(0,caracteres)==c){
formu['lista'].selectedIndex=x;
formu['lista'].style.visibility="visible";
break;
}else{
formu['lista'].style.visibility="hidden";
}
}
}else{
formu['lista'].style.visibility="hidden";
}
}
</script></head>
<body>
<form action="" name="formulario" >
<input type="text" name="buscar" onKeyUp="sel(this.value)" style="width:200px;"><br>
<select name="lista" size="5" style="width:200px;visibility:hidden">
<option value="abc">abc</option>
<option value="abd">abd</option>
<option value="acd">acd</option>
<option value="rcb">rcb</option>
<option value="rbc">rbc</option>
<option value="rbd">rbd</option>
<option value="rbe">rbe</option>
</select>
</form>
</body>
</html>
lo que está en rojo va de yapa, si te sirve, sino , borralo.
saludos