Código PHP:
<html>
<head>
<title>Prueba</title>
</head>
<body>
<form>
<select onchange="if(this.value=='otras'){this.form.descripcion.disabled=false}else{this.form.descripcion.disabled=true}">
<option value="otras">Otras</option>
<option value="unas" selected="selected">Unas </option>
</select>
<input name="descripcion" id="descripcion" type="text" disabled="disabled"/>
</form>
</body>
</html>