JavaScript:

<SCRIPT LANGUAGE="JavaScript1.1" TYPE="text/javascript">
  <!--
  var aryDropdownList = new Array();
  var intArrayCount = 0;
  
  function defineDropdown(strName, strList)
  {
    var objDropdownList = new Object();
    objDropdownList.Name = strName;
    objDropdownList.List = new Array();
    var aryTempList = strList.split("|");
    for (var i = 0; i < aryTempList.length; i++) {
      var aryTempValue = aryTempList[i].split(":");
      objTempValues = new Object();
      objTempValues.Display = aryTempValue[0];
      objTempValues.Value = aryTempValue[1];
      objDropdownList.List[i] = objTempValues
    }
    aryDropdownList[intArrayCount] = objDropdownList;
    intArrayCount++;
  }
  
  function changeDropdown(objTargetElement, strName)
  {
    for (var i = 0; i < aryDropdownList.length; i++) {
      if (aryDropdownList[i].Name == strName) {
        objTargetElement.length = aryDropdownList[i].List.length;
        var intOption = 0
        for (var x = 0; x < aryDropdownList[i].List.length; x++) {
          objTargetElement.options[intOption].text = aryDropdownList[i].List[x].Display;
          objTargetElement.options[intOption].value = aryDropdownList[i].List[x].Value;
          intOption++
        }
        return true;
      }
    }
    return false;
  }
  
  defineDropdown("ALL", "Project Test:1|Project 2:2|Project 3:3|Project 4:4");
  defineDropdown("QA", "Project 2:2");
  defineDropdown("DEV", "Project 3:3|Project 4:4");
  defineDropdown("PROD", "Project Test:1");
  
  //-->
</SCRIPT>

HTML:

<form action="">
<select name="Filter" size="1" onChange="changeDropdown(this.form.Projects, this.options[this.selectedIndex].value)">
 <option value="ALL">All Projects</option>
 <option value="DEV">Projects in Development</option>
  <option value="QA">Projects in QA</option>
  <option value="PROD">Projects in Production</option>
</select>
<select name="Projects" size="1">
 <option value="">____________________________________</option>
  <option value=""></option>
  <option value=""></option>
  <option value=""></option>
  <option value=""></option>
</select>
</form>